To identify risk factors for cerebral toxoplasmosis (CT) in HIV patients (pts) with positive serology for Toxoplasma gondii and CD4 < 200/mm3, data from the placebo group (N = 280) of a primary prophylaxis trial, were analyzed. The probability of onset of CT (n = 46) was, at one year, 13.1%. Three baseline variables were independently associated with a higher risk of CT: group IV non AIDS, AIDS stage and CD4 < 50/mm3. Initial titer of antibodies to Toxoplasma gondii was not.