This has to be the top of the list of reasons for visiting Sabah. The 4,095m mountain is the second highest in Southeast Asia (outside Papua, which is regarded as part of Oceania) after Hkakabo Razi in Burma, but is probably one of the easiest to climb as no mountaineering experience is needed, just a lot of stamina. The surrounding National Park is also home to many plants and animals.
The mountain is sacred to locals. They believe that spirits of their ancestors inhabit the top of the mountain. Previously, a chicken was sacrificed at the peak every time a climb was made but these days this ceremony only happens once a year when only seven chickens are needed to appease the spirits.
Mount Kinabalu is known to be one of the most accessible mountains in the world. No specialized mountain climbing skills are required to ascend it. The trail that most tourists use is described as a 'trek and scramble'. Locals begin climbing the mountain from the age of 3 and the oldest person to reach the peak was 80 years old. However, how much one enjoys the climb depends strongly on how fit you are and how well you acclimatise to the thin air at the higher levels.
Nevertheless, the mountain can be a dangerous place, especially during the rain or when there is mist. On average, every year one person gets into severe difficulty out of the estimated 20,000 people who attempt the climb. The higher slopes can be very slippery when it rains and dense fog reduces visibility to a few feet.
Although it is possible to climb to the top and back in less than four hours, most climbers take two days, with an overnight break at Laban Rata (3,272.7m above sea level). The final attack on the peak takes place in the early hours of the second day (most begin at 2:30AM) in order to catch the sunrise at the top. By mid-morning the mist begins to roll in, obscuring the breath-taking views.
Climbing weather is best around the month of April while November and December brings rain. The temperature ranges from a comfortable 20-25°C at the main park to something approaching freezing near the top (depending on the weather). Bring clothing appropriate or else you will get cold and be miserable. If possible, climb during the full moon as it helps illuminate the white rope that marks out the climbing path.
Climbing the mountain is strongly encouraged, but be forewarned that it can be strenuous. Also, it is best to book accommodation beforehand — indeed, the hike has become so popular that you may have to book months in advance. The park can be very full especially during clear weather periods. Note that it is not possible to stay overnight on the mountain except in the accommodation provided at Laban Rata, and camping is not permitted - you must, therefore, have pre-booked accommodation. (See Sleep for details.) However, sometimes there are cancellations. If you are unable to book mountain hut accommodation in advance, get to the Park HQ as early as possible on the day of your intended climb and inquire there. You may luck out and get a bed. By regulation, booking must include guide services.
As you struggle up the mountain, do look out for porters laden with 14 kilogram gas cylinder tanks. Some wear slippers and most have jury-rigged harnesses (some made of rice sacks) piled high with supplies. The porters scale the 6.5km distance between Timpohon Gate and Laban Rata resthouse several times a day on supply runs and can easily outpace better-equipped climbers who are unfamiliar with the trail.
Once you complete the strenuous hike up to 11,000ft, you might have enough energy to do the only via ferrata ("iron road", a set of cables and ladders bolted to the mountain)  to be found in Asia, which happens to be the world's highest. It is still relatively unknown so take the opportunity to do it without a crowd of people pushing you. There are 3 different paths to take on the via ferrata, with times ranging from 4-5 hours for the longest to 2-3 for the shortest. Keep your travel plans in mind when booking the via ferrata and after-mountain activities, as this time is added to the 4-5 hours it takes to descend the mountain on the second day of hiking.
For the more advenurous and physically fit, there is also an option to climb to the summit and down in one day. This is not widely publicised - presumably to maintain patronage at the expensive mountain huts. To arrange this, you should speak to the park ranger at the park office - turning up in person is generally more effective, so that the ranger can see that are serious and is less likley to fob you off than if you were to enquire by phone. The one day climb is more weather dependend than the two day option, and there are set time limits for each stage of the climb which must be met, otherwise the rules say you will be turned around (it's not clear whether this actually happends, but be aware that the time limits would require you to climb at a reasonable pace). You will be required to hire a guide. You can expect to start at 0730 and will be required to come down by 1730 before the Timpohon Gate is locked. Because the one day climb will bring you to the summit well after sunrise, you should consider the sesonal weather patterns - at times, the mountain top clouds over shortly after sunrise and you may not get a view from the summit. The climb to the summit is almost 2300m, and the one day climb is physically tough. Nevertheless, it is achievable, and during the Mt Kinabalu Climbathon top runners complete this route in under 2 hours 40 mintues.