We measured the association of response to treatment, assessed as changes in CD4+ T cell counts after antiretroviral therapy, of five polymorphisms in four genes (CYP2B6, ABCB1/MDR1, ABCG2, and ABCC4) in which variation has been suggested to affect the pharmacokinetics of drugs commonly employed to treat HIV/AIDS. This evidence concerns the gene ABCG2 and AIDS.