Twenty-eight infants and children hospitalized for severe shigellosis were treated orally either with ampicillin trihydrate (100 mg/kg/day administered in divided doses every six hours) or with trimethoprim-sulfamethoxazole (trimethoprim, 10 mg; sulfamethoxazole, 50 mg/kg/day in divided doses every 12 hours) for five days. Four patients with ampicillin-resistant shigellae continued to have diarrhea and positive stool cultures during therapy. Patients with susceptible shigellae treated with ampicillin and all patients treated with trimethoprim-sulfamethoxazole responsed promptly and comparably within an average of 1.6 and 1.7 days, respectively, until stool cultures were negative, and 3.1 and 2.9 days, respectively, until diarrhea stopped. Patients with ampicillin-resistant shigellae responded to treatment with trimethoprim-sulfamethoxazole. It is concluded that trimethoprim-sulfamethoxazole is the best currently available drug for treatment of shigellosis in areas where multiple antibiotic resistance of shigellae is common.