To determine HIV-1 subtypes in Korean patients, we analyzed the nucleotide sequences of the HIV-1 env gene isolated from 19 Korean patients. DNA was extracted from cultured lymphocytes and the proviral V3 env gene was amplified by nested polymerase chain reaction. DNA sequences were determined using a cycle sequencer with fluorescence-labeled dye terminators, and aligned using a set of reference sequences. The phylogenetic tree was constructed by the neighbor-joining method. Most of the enrolled patients were in the advanced stage of AIDS with CD4+ lymphocyte counts ranging from 10 to 560 (median 50) per mm3. Eighteen of the nineteen patients' sequences fall into subtype B (95%) and one was subtype A (5%). The tetrameric motifs at the tip of the V3 loop were comprised of GPGR (10 cases, 53%), GPGS (5 cases, 26%), and GPGQ, GPGG, GQGR, and APGS (one case each, 5%). Subtype B was found to be the most predominant clade of HIV-1 in our AIDS patients.