To investigate resistance to lamivudine (3TC), we examined the incidence of M184V in 20 HIV-1 patients treated with 3TC for $13.1{pm}9$ months. Fourteen of 20 patients had been exposed to zidovudine (ZDV) or didanosine (ddI) prior to 3TC therapy. Nested PCR targeting to reverse transcriptase (RT) and direct sequencing were performed for peripheral blood mononuclear cells sampled serially. There were resistance mutations to ZDV in at least 9 patients at baseline, although there was no resistance mutation to 3TC. We could detect M184V in 6 (30%) out of 20 patients. The incidence of M184V increased as the duration of therapy prolongs (13% in samples <12 months; 47% in samples ${ge}12$ months). The frequency of mutation M184V was higher in patients with previous mutation to ZDV than in patients with wild type. Resistance mutation was not detected in 7 patients. This study shows that resistance to 3TC tends to develop rapidly in patients with baseline mutations or two drugs combination therapy than in those treated simultaneously with triple drugs. This report is the first on resistance to 3TC in Korean AIDS patients.