Cyclospora is a coccidian parasite that infects the upper intestine and causes a prolonged illness consisting of fatigue, anorexia, and diarrhea. Untreated infections can last for several weeks.1 Trimethoprim-sulfamethoxazole (co-trimoxazole) was found to be an effective treatment for Cyclospora infections in a 1994 study performed in Nepal.2 However, people with known allergies to sulfa drugs cannot take co-trimoxazole. A number of antibiotics have been tried against Cyclospora infections without success, including norfloxacin, tinidazole, diloxanide furoate, and quinacrine hydrochloride. Azithromycin was not successful in a small open trial in 1993.3 Trimethoprim is not chemically related to sulfa, and allergy to co-trimoxazole is usually attributed to the sulfamethoxazole component. In order to find a treatment for people infected with Cyclospora who are allergic to sulfa drugs, we undertook an open trial of trimethoprim alone, in a dose of 200 mg twice a day for 7 days.