It has been hypothesized that coinfection with mycobacteria occurs in patients with Pneumocystis carinii choroiditis, but cases demonstrating ocular infection by both organisms have not been reported. This study reports the case of a patient with P. carinii choroiditis who was treated with intravenous trimethoprim and sulfamethoxazole, followed by intravenous trimethoprim and dapsone. The choroidal lesions failed to resolve despite 6 weeks of treatment, and the patient died from massive pulmonary infection caused by P. carinii, Mycobacterium avium-intracellulare, and cytomegalovirus infections. Ocular histologic and electron microscopic examinations revealed choroidal infection by both P. carinii and M. avium-intracellulare. Serum levels of sulfamethoxazole were below the recommended therapeutic range for treating P. carinii infection during the first week of therapy, but adequate drug levels were subsequently obtained. Failure of choroidal lesions of P. carinii to resolve in some cases may suggest insufficient antimicrobial levels in the blood or raise the possibility of coexistent M. avium-intracellulare or other opportunistic infection.