BACKGROUND: Limited data are available on genetic subtypes of HIV- 1 in Korean AIDS patients. To determine subtypes of HIV-1 in Korean patients, we analyzed nucleotide sequences of the env gene of HIV-1 and constructed a phylogenetic tree. METHODS: Nineteen patients infected with HIV-1 were enrolled. The median CD4 + count was 85/mm 3. Peripheral blood mononuclear cells (PBMC) were collected and co-cultivated with pre-stimulated PBMC from HIV-seronegative donors for 7 ~14 days. DNA was extracted from cultured lymphocytes and proviral V3 region of the env gene was amplified by nested polymerase chain reaction (PCR) for direct sequencing without cloning. Sequence analysis was performed by cycle- sequencing and dye terminator methods with an automated DNA Sequenator. The sequences were aligned with nines ets of reference sequences for each subtype by Clustal method. Phylogenetic tree was constructed by the neighbor j oining method. RESULTS: Eighteen of the nineteen sequences fell into subtype B (95%) and one was subtype A (5%). The patient nfected with subtype A was an ex-prostitute and had engaged in sexual contact with sailors who are generally regarded as one of the highest risk groups of HIV infection in Korea. The tetrameric motifs at the tip of the V3 loop were comprised of GPGR (six cases, 32%), GPGS (three cases, 16%), GPGQ, GPGG, GPGK, APGS (one case each, 5%) CONCLUSION: Subtype B is predominant clade of HIV-1 isolated from Korean patients and only one case showed subtype A.