The presence of HIV-1 antibodies was determined among the injecting drug users (IDUs) and their non-injecting wives. A total of 233 (72%) were found to be HIV-1 seropositive among the 322 subjects recruited in this study between August, 1996 and September, 1997. The distribution of HIV-1 subtypes among the injecting drug users (IDUs) and their wives was determined using peptide enzyme immunoassay (EIA). Sexual transmission of HIV-1 occurred frequently (45%) from HIV-1 infected IDUs to their spouses. The majority of the subjects (167/233) were infected with subtype C followed by subtype Thai B (29/233). Subtype C was the most common among both IDUs (78%) and their wives (57%), followed by subtype Thai B (12% and 13% respectively). The distribution of subtypes was significantly different between IDUs and their wives with a lower percentage of subtype C and higher percentage of subtype D in the infected wives (P < 0.03). Discordance for subtypes transmitted from IDUs to their wives suggests the occurrence of dual and/or recombinant infection in the IDUs.