Resting energy expenditure and nitrogen balance were measured shortly after admission and shortly before discharge or colectomy in eight adult males hospitalized for active ulcerative colitis. The measured resting energy expenditure was 19% higher than that predicted by the Harris-Benedict formula (p less than 0.01). Fecal and urinary nitrogen losses were 2.1 +/- 1.5 g/day and 13.6 +/- 3.8 g/day, respectively. During the early hospitalization period, daily intakes of calories (1,826 +/- 1,050 kcal) and protein (1.5 +/- 0.4 g/kg) were not sufficient to maintain energy (-548 +/- 964 kcal/day) and nitrogen (-1.5 +/- 3.9 g/day) balances. Energy expenditure and nitrogen losses at the end of the study were similar to the values obtained on admission, but because of a voluntary increase in caloric (by 42%) and protein (by 37%) intake during the hospitalization both energy balance (+434 +/- 409 kcal/day) and nitrogen balance (+4.6 +/- 6.5 g/day) (p less than 0.05) improved. These results suggest that, although the metabolic impact of active ulcerative colitis is mild, it does result in negative energy and nitrogen balances. Dietary intake can be voluntarily increased during hospitalization to improve energy and nitrogen equilibrium.