The features of the nine first cases of Isospora belli enteritis in patients with acquired immunodeficiency syndrome (AIDS) diagnosed in the Hospital Cliníc i Provincial of Barcelona from September 1984 to May 1989 are reported. All patients were male, five were homosexual and four were parenteral drug abusers. The clinical presentation was watery diarrhea without pathological products lasting for more than one month. Five patients had features of dehydration, five had malabsorption, two had fever and one had metabolic acidosis. Enteritis by I. belli was the first opportunistic infection in eight of the nine cases. The number of T4 lymphocytes was lower than 0.4 x 10(9)/l in four of the seven patients in whom it was measured, and the p24 antigen was detected in serum in three out of five. The response to co-trimoxazole, both in the acute phase and as maintenance therapy, was satisfactory; however, two patients had recurrences despite maintenance therapy with co-trimoxazole. In one of them I. belli was clinically resistant to co-trimoxazole therapy and to other drugs, the diarrhea only responding to the administration of a somatostatin analogue (SMS 201-995).