OBJECTIVE: To report a cohort of patients in whom polymerase chain reaction (PCR) was performed on vitreous samples and to place in perspective the current role of PCR in the diagnosis of ocular toxoplasmosis. DESIGN: Noncomparative case series. PARTICIPANTS: Fifteen patients in whom toxoplasmic retinochoroiditis was considered in the differential diagnosis and in whom the clinical presentation was not diagnostic and/or response to treatment was inadequate. INTERVENTION: Examination of vitreous fluid by PCR and of serum for the presence of Toxoplasma-specific antibodies. MAIN OUTCOME MEASURES: Presence of Toxoplasma gondii DNA, serologic test results, clinical findings, treatment, and outcome. RESULTS: In 7 of 15 patients, vitreous fluid examination results by PCR were positive for the presence of T. gondii DNA. Five of these seven patients had serologic test results consistent with Toxoplasma infection acquired in the distant past; the other two patients had serologic test results consistent with retinochoroiditis in the setting of acute toxoplasmosis. The PCR results influenced the management of these patients in six of the seven positive cases. In the eight patients in whom vitreous examination results were negative by PCR, either Toxoplasma serology was negative (6), the retinal lesions were caused by cytomegalovirus (1), or, on further consideration, the eye signs were not consistent with those of toxoplasmic retinochoroiditis (1). CONCLUSION: In patients in whom toxoplasmosis is considered in the differential diagnosis but in whom the presentation is atypical, PCR was frequently a useful diagnostic aid.