Resistance to antiretroviral drugs is generally conferred by specific amino acid substitutions, rather than insertions or deletions, in reverse transcriptase (RT) of human immunodeficiency virus type 1 (HIV-1). The exception to these findings is the amino acid insertions found in the beta3-beta4 loop of the RT enzyme in response to treatment with nucleoside reverse transcriptase inhibitors. This insert consists most commonly of two amino acids, but we describe in detail the evolution of a variant with an 8-amino-acid (aa) insert in a patient treated with zidovudine (ZDV) and 2'-3'-dideoxycytidine (ddC). The 24-nucleotide insert is a partial duplication of local sequences but also contains a sequence segment of unknown origin. Extensive sequence analysis of longitudinal patient samples indicated that the HIV-1 population prior to the start of therapy contained not the wild-type amino acid 215T in RT but a mixture with 215D and 215C. Treatment with ZDV and subsequent ZDV-ddC combination therapy resulted in the evolution of an HIV-1 variant with a typical ZDV resistance genotype (41L, 44D, 67N, 69D, 210W, 215Y), which was slowly replaced by the insert-containing variant (41L, 44D, insert at position 69, 70R, 210W, 215Y). The latter variant demonstrated increased resistance to a wide range of drugs, indicating that the 8-aa insert augments nucleoside analogue resistance. The gain in drug resistance of the insert variant came at the expense of a reduction in replication capacity when assayed in the absence of drugs. We compared these data with the resistance and replication properties of 133 insert-containing sequences of different individuals present in the ViroLogic database and found that the size and actual sequence of the insert at position 69 influence the level of resistance to nucleoside analogues.