Two anonymous population-based surveys were conducted between August 2001 and April 2002 using computer-assisted telephone interviews of 3793 male and 3244 female randomly selected residents of Hong Kong in order to estimate the prevalence of psychotropic substance use and identify risk factors and associations with health outcomes among users in Hong Kong. The study cohort was comprised of ethnically Chinese adults between the ages of 18 and 60 years of age. In the last 12 months, 5% of males and 1.8% of females had used psychotropic substances such as cannaboids, methylamphetamines, ketamines, cocaine, opiates, barbiturates, and benzodiazepines. Of the male drug users, almost one-third had done so outside of Hong Kong. "Risk factors" for substance use included: younger age, unemployment, being an early school leaver, being unmarried, "heavy alcohol consumption," and smoking. Psychotropic substance use in the past year was strongly associated with increased prevalence of long-lasting (> or =3 months) sexual health problems during the past year and with a higher prevalence of HIV/STD risk behaviors. The prevalence of substance use in Hong Kong has been under-reported. Although significant correlations were noted between substance use and various respondent attributes, the directionality of the associations needs to be determined with longitudinal studies. It may be prudent to integrate substance use and HIV/STD prevention efforts.