RESUMEN
Subtypes B' and E are the two major subtypes of HIV-1 among injecting drug users (IDU) in Thailand. However, there are not many reports on subtype distribution during the early epidemic. Random blood specimens collected during 1994-2000 from 3,286 IDU at the Thanyarak Hospital were tested for HIV antibody and subtyped by using peptide binding enzyme immunoassay. The prevalence rate of HIV infection was 36.8%. All HIV-seropositive IDU were ascertained for "year of first HIV seropositivity" from their medical records. Of 1,512 HIV-seropositive samples, 1,408 (93.1%) were typeable. During 1987-1988, the proportion of subtype B' was as high as 80% but decreased rapidly to 27.6% during 1999-2000. At the same time, the proportions of subtype E increased correspondingly (Chi-square test for the trend, p < 0.05). The relatively high proportion of subtype E among IDU since an early stage of the epidemic suggests early co-existence of both subtypes and needs further investigation.