Human immunodeficiency virus (HIV) has spread widely among injecting drug users (IDUs) in countries to the north and west of the 'Golden Triangle' region of South-East Asia; it is likely to have spread southwards to Malaysia as well. In order to assess HIV seroprevalence among IDUs in north-east Malaysia and describe risk factors for HIV infection in this population, we performed a cross-sectional seroepidemiological study among 210 IDUs recruited at the detoxification ward of the General Hospital in the capital city of the north-eastern Malaysian state, Kelantan. Subjects were sequential entrants to the detoxification ward, interviewed about HIV risk behaviour, and tested for antibody to HIV and to syphilis. Nearly a third (62/210, 30%) of these IDUs were HIV seropositive. Three-quarters (159/210) had travelled to Thailand in the preceding 5 years, of whom 32% (51/159) were HIV seropositive; this was associated with injecting in Thailand, but not with sexual contact there. Of those who had not left Malaysia in the preceding 5 years, 26% (11/43) were HIV seropositive, a rate not significantly different from those who had travelled. Travel within Malaysia was common (144/210, 69%) among IDUs interviewed, as was unsafe injecting and unsafe sexual behaviour (20% had shared injecting equipment and 21% had had unprotected intercourse) in other states. In every locale, rates of unsafe injecting behaviour were high (55% sharing in last month), even among those who knew they were HIV infected, and rates of condom usage were low (93% of 160 sexually active IDUs had never used a condom). Syphilis was not associated with HIV infection, but with contact with Thai prostitutes.(ABSTRACT TRUNCATED AT 250 WORDS)