OBJECTIVE: To identify the risk factors for high altitude pulmonary oedema. DESIGN: A case control study. PLACE AND DURATION OF STUDY: The study was conducted in the Department of Medicine, Military Hospital, Rawalpindi during October 1998 to October 2000. SUBJECTS AND METHODS: One hundred serving army personnel between 20 to 50 years of age were divided in two groups. First group comprised of 50 patients who developed high altitude pulmonary oedema at 4000 meters height and above. Second group consisted of 50 control subjects who accompanied the patients. Detailed history was obtained regarding height of native dwelling, rate of ascent, amount of exertion and preceding respiratory tract infection. The findings of both the groups were compared and statistically analyzed by application of Pearson's chi-square test. Difference was considered significant when p-value was < 0.05. RESULTS: All the subjects were male. Average age of the indexed group was 29.4(+/- 4) years as compared to 27(+/- 5) years in the control group. Six patients (12%) were highland dwellers whereas 44 patients (88%) were inhabitants of lower heights. Average rate of ascent from 4000 meters to 4500 meters of the indexed group was 5(+/- 2) days while in control group it was 10(+/- 3) days. Twenty cases (40%) had history of severe exertion whereas such history was not present in any of the control group subjects. Only 2 patients had signs and symptoms of preceding respiratory tract infection. CONCLUSION: Rapid rate of ascent, extreme physical exertion, low height of native dwelling and preceding respiratory tract infection are important risk factors for the development of high altitude pulmonary oedema. Proper acclimatization with gradual ascent plays important role in preventing this potentially fatal condition.