Introduction: Acute Mountain Sickness (AMS) is a high-altitude medical emergency that requires prompt treatment. If left untreated AMS can progress to high-altitude cerebral edema or pulmonary edema, both of which can be fatal. As the popularity of high altitude trekking increases in the Himalayas we were interested in determining what rates of AMS are on popular routes in this region. Methods: AMS was diagnosed using a standardized Lake Louise Symptom Score (LLSS) where scores 3-5 denoted mild AMS and >5 denoted severe AMS. Forms were distributed to trekkers prior to departure and symptoms scores were determined daily. Data on medical history and patient demographics were also collected. All data are expressed as mean ±SEM. Results: Preliminary results are reported from N=17 (4 female) participants. Mean age was 43.7±3.9y. Most subjects, 68.8%, had trekked above 2500 m in the past. Only 6.25% reported having no knowledge of AMS, with the others having limited or expert knowledge. 25% of subjects had previously suffered from AMS. Most subjects, 82.4%, took prophylactic AMS medication, acetazolamide; at a dose of 250 mg/d. Subjects trekked at a mean altitude of 3650±85 m and ascended to a maximum altitude of 5012±103 m. The mean LLSS was 1.48±0.31 with a maximal LLSS of 4.76±0.75. Within our sample, 70.86% suffered from AMS at some point during their trek. Of those who suffered from AMS, the mean number of days affected was 3.17±0.61, and of those with severe AMS, mean number of days affected was 2.14±0.7. Conclusion: Over 70% of trekkers to the Himalayas experience AMS for an average of 3d, despite the use of prophylactic medication that most participants take. Almost 95% of trekkers have working knowledge of AMS and most have prior experience trekking at high-altitude. Given the dangers of high altitude trekking, pre-departure education for patients, especially those with chronic diseases, alongside prophylactic medication for AMS may help mitigate the risk.