Reactivation of chronic, latent infection of Toxoplasma gondii has been observed previously in transplant patients and malignancies treated with immunosuppressive drugs. Recently it has emerged as the most common recognized central nervous system infection in acquired immunodeficiency syndrome (AIDS) patients, seen in 6-20% of all AIDS cases. It produces diffuse encephalopathy or space-occupying and necrotic lesions. Contrary to classical toxoplasmosis, the serological diagnosis is unreliable and the therapeutical response in most cases is poor.