We sought evidence of toxoplasma parasitemia among 37 people with active or dormant Toxoplasma gondii infection or no evidence of infection. DNA was extracted from erythrocyte-free portions of blood samples, and the T. gondii B1 gene was amplified by the polymerase chain reaction. Evidence of T. gondii parasitemia was found in six patients with severe immunosuppression from AIDS and clinical evidence suggestive of or compatible with toxoplasmosis. Results were negative for patients unlikely to have active toxoplasmosis. Gene detection after amplification with the polymerase chain reaction is a promising test for detection of parasitemia, and parasitemia should be tested for in patients with AIDS and unexplained fever or central nervous system abnormalities.