The effect of proctocolectomy and small bowel resection on working capacity has not been assessed objectively in previous research. Twenty-nine patients with Crohn's disease were investigated with cycle ergometry and a questionnaire, following proctocolectomy with and without small bowel resection. Maximal exercise load is known to correspond well with working capacity, particularly when account is taken of body composition and metabolic variables. Maximal exercise load was reduced marginally (by 9 per cent) in patients without small bowel resection and by 22 per cent in patients with moderate small bowel resection (15-30 per cent resection). Patients with extensive bowel resection (more than 50 per cent) had a 40 per cent reduction in the maximal exercise load. This reduction in maximal exercise load was greater than predicted when accounting for reduction in muscle mass. All patients had a normal oxygen uptake including resting energy expenditure. Urinary sodium and magnesium excretion was low in the group with moderate bowel resection, whereas the extensively resected patients were malnourished and had a reduced body cell mass. The authors conclude that the significantly reduced working capacity was of multifactorial origin secondary to malabsorption. However, the patients seemed unaware of the degree of their diminished working capacity. This reduced capacity makes it unlikely that they would be able to perform any labour involving high energy consumption at the level of 500-700 W, and this inability was reflected by a high rate of unemployment among the patients.