JC virus (JCV) is thought to reach the central nervous system by a vascular route. To determine whether JCV is conveyed in peripheral blood as latent or reactivated virus, blood leukocytes, plasma, and urine from 50 AIDS patients and plasma and B lymphocytes from 60 AIDS patients were investigated. Peripheral blood from 88 human immunodeficiency virus-negative blood donors was studied. Nested polymerase chain reaction assays allowed the identification of JCV T DNA and VP1 mRNAs. The latter indicate viral replication. Blood harbored JCV DNA in 31.8% of AIDS patients (only 2.3% of blood donors; P > .001) and urine in 56%. VP1 mRNAs were detected in blood of 1 AIDS patient. Notably, 38% of DNA-positive urine samples and 10 cerebrospinal fluid samples (CSF) from AIDS patients with progressive multifocal leukoencephalopathy contained JCV mRNAs. Thus, JCV was significantly more frequent in blood from AIDS patients than from controls, but, in most instances, it was latent, whereas active replication was detected in urine and CSF.