In an attempt to study the frequency and distribution of human herpesvirus-6 (HHV-6) infection both in normal and neoplastic brain tissues in vivo, polymerase chain reaction was used to look for HHV-6 genomes: 1) in samples, obtained at necropsy, from different regions of the brain of immunocompetent adult subjects and of patients who died of AIDS; 2) in the surgical biopsies of a well-characterized series of primary brain tumors of neuroglial origin. HHV-6-specific sequences were identified in six of nine brain samples from immunocompetent subjects, and in four of seven brain samples from AIDS patients. Viral sequences were identified in the specimens derived either from the grey (frontal cortex and basal ganglia) or from the periventricular white matter. HHV-6 DNA was found only in 6 of the 37 primary brain tumor biopsies examined. This study provides for the first time molecular evidence of a wide distribution of HHV-6 infection in the brain tissues of a high proportion of subjects, both in normal and in impaired immunity. In this large series of tumor biopsies the presence of HHV-6 genomic sequences is a rare phenomenon, arguing against a major role of this herpesvirus in the pathogenesis of primary brain tumors of neuroglial origin in immunocompetent subjects.