Weight loss and changes in body composition are recognized phenomena associated with high-altitude mountaineering expeditions. Attempts to partition the weight loss between fat mass (FM) and fat-free mass (FFM) have been inconclusive. Therefore, five male subjects, average age 40.0 +/- 5.5 years, were studied prior to, during, and following a 21-day expedition between 2200 m and 4300 m on Mt. McKinley, Alaska. Pre- to postexpedition body composition changes were determined by densitometry, skinfold thickness (12 sites), body girth measurements (14 sites), and cross-sectional area from magnetic resonance imaging (MRI) (three sites). Data analysis by Student's t-test (p < 0.05(1) tail) indicated decreases in body weight (4.2 +/- 2.8 kg, 220 g/day, 5.4% of initial weight), total skinfold thickness (10.8%), total body girth (2.8%), and percentage of fat by densitometry (15%). Total cross-sectional area of the three MRI slices (upper arm + thigh + calf) decreased 4.7%, muscle area decreased 9.1%, and fat area decreased 3.9%. The three methods used to partition the weight loss between FM and FFM did not agree: 77% FM vs 23% FFM by densitometry, 25% FM vs 75% FFM by skinfolds, and 38% FM vs 62% FFM by MRI. Energy intake (3640 +/- 1250 kcal/day) was negatively correlated (Pearson r > 0.88(2) tail) with losses in weight (r = -0.89), skinfolds (r = -0.93), and girths (r = -0.88), ie the greater the intake the less the decline. Therefore, insufficient energy intake appeared primarily responsible for the weight loss and changes in body composition during the expedition.