The development of antiretroviral drug resistance is a major threat to the effective treatment of human immunodeficiency virus type 1 (HIV-1) infection. Drug treatment failure is associated with accumulation of drug resistance mutations and the evolution of drug resistance. Studies from microbial systems provide evidence for a correlation between drug resistance development and increased pathogen mutation rates. Recent studies with HIV-1 have shown that drugs targeted against reverse transcriptase (RT) as well as drug-resistant RT can increase HIV-1 mutation frequencies. Furthermore, combinations of drug and drug-resistant RT have been found to increase virus mutation frequencies in a multiplicative manner. The correlation of increased HIV-1 mutation rates with the evolution of antiretroviral drug resistance indicates that drug failure could increase the likelihood of further resistance evolving from subsequent drug regimens.