Beneficial effects of GB virus-C (GBV-C) on HIV disease reported by some investigators could not be observed by others. We therefore asked if GBV-C altered CD4 cell count in HIV co-infected individuals in two Kenyan cohorts: A commercial sex workers (CSW) cohort and an antenatal mother-child health (MCH) cohort. Anti-HIV antibodies were detected, in archived plasma samples from the Kenyan cohorts, using enzyme-linked immunosorbent assay (ELISA). CD4 and CD8 counts in whole blood were obtained using Tritest flow cytometry assay. Active GB virus-C infections were determined by polymerase chain reaction (PCR). Data were analyzed using Microsoft Excel, SPSS, and Epi Info statistical packages. Positive HIV serology with CD4 cell count of 200 cells/µl or less, at two consecutive and continuing determinations was regarded as suggestive of progressing HIV disease. More than half of the women in the sample were HIV-positive and about one-fifth were GBV-C positive. The risk of being HIV positive was significantly greater among the CSW cohort compared to the MCH cohort. HIV positive women co-infected with GBV-C had similar CD4 levels, compared to HIV positive women negative for GBV-C. These data provide no evidence that GBV-C co-infection protects against decline in CD4 cell counts in these cohorts.