The effect of an H2-receptor blocking agent, cimetidine, on faecal losses of fluid, electrolytes and fat was examined in 10 patients with Crohn's disease, who had diarrhoeas after extensive small bowel resection. A randomized, double-blind and cross-over design was applied, and patients were hospitalized and on a defined diet during the study. Cimetidine, 4 x 400 mg, significantly reduced diarrhoeal volumes by an average of 22% (p less than 0.05) and faecal sodium by 27% (p less than 0.05). Patients with severe diarrhoeas responded better to treatment. No side-effects were recorded. The reported data suggest that cimetidine may be useful in symptomatic treatment of patients with severe diarrhoeas after extensive ileal resection. Due to deficient drug absorption, higher doses may be needed for optimal effect.