Sexual risk behaviour, measured by anonymously self-reported sexual activity, number of sexual partners, and condom use rates, was studied in age- and gender-matched HIV-1 positive consecutive patients from the Thai Red Cross Immune Clinic (IC) (study group, questioned after voluntary HIV counselling and testing) and the Thai Red Cross Anonymous Clinic (AC) (control group, questioned before voluntary HIV counselling and testing) in Bangkok, Thailand in 1993/94. More than 80% of study patients reported having decreased their sexual activity and their number of sexual partners since receipt of the positive HIV test result. Compared to control patients, study patients reported more often abstaining from sex (42% vs 14%), and more often using condoms during all their last three incidences of sexual intercourse (44% vs 14%). These findings give evidence for the value of voluntary HIV counselling and testing in contributing to the reduction of HIV transmission in Asia.