Two AIDS patients were provisionally diagnosed as having cerebral toxoplasma infection on the basis of compatible clinical presentation, serological evidence of exposure to toxoplasma and the detection of multiple space occupying lesions on CT scan. Initial response to conventional antitoxoplasma therapy was poor. Brain biopsy was performed and toxoplasma nucleic acid detected in the cerebral tissues utilising the polymerase chain reaction (PCR). Specific therapy was continued and a satisfactory clinical and radiological response was achieved in each case. The PCR represents a method of potential value for the diagnosis of cerebral toxoplasmosis associated with AIDS. Further studies are required to assess the sensitivity, specificity and prognostic value of this technique in comparison with established diagnostic methods.