The purpose of this study was to assess the incidence of medical illness among members of trekking groups in the Nepal Himalaya. The design was a cohort study using interview and clinical examination by a single physician. The setting was the Manaslu area in the central Nepal Himalaya along a 22-day trekking route with elevations ranging from 487 m to 5100 m. Subjects were 155 members of commercial trekking groups: 102 Nepali porters, 31 Nepali trek staff, and 22 Western trekkers. We found that medical problems occurred in 45% of party members. The porter cohort contained the highest diversity and severity of illness. The relatively larger porter cohort experienced 77% of the medical problems recorded compared with 17% among Western trekkers and 6% among trek staff. The incidence of medical problems was not significantly different in the porter staff (52%) and Western trekkers (55%) and was significantly lower for the trek staff (13%). High-altitude pharyngitis/bronchitis was the most common illness in the party (12%) followed by acute mountain sickness (8%) and gastroenteritis (6%). Other conditions included anxiety (3%), cellulitis (3%), scabies (3%), snow blindness (3%), acute alcohol intoxication (2%), conjunctivitis (2%), fever (2%), lacerations (2%), and hemorrhoids (1%). Illness with infectious etiologies comprised 33% of the medical problems. The incidence of altitude illness was not significantly less in the Nepali porter staff than in the Western trekkers. Evacuation was required in 5% of party members, all from the porter group. This study should alert expedition medical providers and trip leaders of the need to be observant for and prepared to treat the frequent and diverse medical problems among the porter staff in their party, in addition to the Western members. Medical problems are common in remote mountainous areas, indicating that trip physicians should be experienced in primary care.