Neurological complications occur in 40% of "human immunodeficiency virus type 1" (HIV-1)-infected patients. Aim of the study was to evaluate the diagnostic yield of stereotactic brain biopsy and non invasive diagnostic procedures (CT, antitoxoplasma antibodies) and to calculate the benefit of the brain biopsy for the patient and the costs of both methods. From October 1989 through September 1995 we biopsied 44 of 2749 (2%) HIV-1-infected patients after non invasive diagnostic procedures had been performed. In 93% of the patients an unambiguous diagnosis was possible based on the biopsy and lead in 73% of the patients to a change of therapy. No complications occurred after biopsy. 40 CTs and 15 MRIs were done. The radiological appearance of toxoplasmosis and non Hodgkin lymphoma (NHL) differed from that of progressive multifocal leucencephalopathy (PML) in respect to enhancement (PML). CT showed a sensitivity of 55% (toxoplasmosis, NHL) and 78% (PML) and a specificity of 83% (PML), 84% (NHL) and 96% (toxoplasmosis), respectively. Antitoxoplasma antibodies showed a sensitivity of 45%, only. The stereotactic brain biopsy was more expensive (20.166,- ATS) than CT, MRI and antitoxoplasma antibodies (4109,- ATS up to 6959,- ATS). We conclude that stereotactic brain biopsy is an efficient and safe and for the patients important diagnostic procedure. In selected patients even expensive investigations should be undertaken considering specific therapy and cost effective homecare.