To study how GB virus C (GBV-C) coinfection affects the response to highly active antiretroviral therapy (HAART), 146 human immunodeficiency virus (HIV)-infected patients were tested for GBV-C RNA and antibodies against GBV-C E2 protein, and responses to HAART were evaluated. GBV-C-infected patients exhibited a complete virological response to HAART more often than patients without [correction] GBV-C and had a greater increase in median CD4 cell count and a marginally greater median HIV RNA level decrease. This association was found to be independent of baseline CD4 cell count and plasma HIV RNA, which indicates that an association exists between GBV-C infection and response to HAART.