Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease of the central nervous system, which is thought to be a result of the reactivation of JC virus (JCV), a human polyomavirus. The disease occurs in individuals with immunosuppression and in recent years there has been an increase in PML cases due to AIDS. A nested polymerase chain reaction (n-PCR) was employed to detect JCV and BK virus (BKV) DNA in brain tissue collected postmortem from 28 AIDS patients with PML and from 13 patients without PML, but with other diagnoses, including solid tumors, Alzheimer's disease, thromboembolism, myocardial infarction and acute cerebrovascular diseases. All 28 brain specimens from the patients with PML were positive for JCV DNA when tested by n-PCR and three of the latter were also positive for BKV DNA. These results were confirmed by an enzyme restriction analysis and a DNA hybridization assay. Interestingly, in this study, JCV DNA was also found in 6 brain tissue specimens from 4 subjects with diseases unrelated to PML or AIDS. All the brain specimens from the control group were negative for BKV DNA. The results confirm that the n-PCR is a useful tool for PML diagnosis. The presence of JCV DNA in the brain tissue of patients without PML is particularly important since it indicates that JCV could be latent in the brains of immunocompetent individuals. Moreover, detection of simultaneous presence of JCV and BKV in the brain tissue of the patients with PML demonstrates that BKV may also infect the human brain without causing any apparent neurological disease.