We describe patients with and without AIDS who had Cyclospora cayetanensis infection; these patients were seen at a tertiary care teaching hospital in Mexico City because of diarrheal disease. C. cayetanensis was detected by examination of fresh fecal preparations and acid-fast staining of fecal smears; the presence of other bacteria and parasites was excluded by standard methods. Fecal specimens from 12 patients contained C. cayetanensis. The overall mean duration of illness was 94 days. Seven of the 12 patients had AIDS; these patients presented with more weight loss than did patients without AIDS (P = .04). The patients with AIDS also tended to have a more prolonged illness. Two patients with AIDS had biliary disease that resolved when they received therapy with trimethoprim-sulfamethoxazole for cyclospora infection; the excretion of oocysts also ceased. Our data confirm that C. cayetanensis causes diarrhea in humans and a significant weight loss in patients with AIDS. In addition, Cyclospora could be involved in biliary disease in patients with AIDS.