This study describes high-risk behaviour and condom usage in inhabitants of an urban slum in the Union Territory of Chandigarh, which has a population of about 20,000. A cohort of 375 participants was followed-up before and after an intervention. Half of the married but only one-eighth of the unmarried slum dwellers were using condoms regularly. One-quarter of the study subjects practised high-risk behaviour and this was higher among unmarried (44.88%) than married participants (18.7%). An intervention involving education on condom use and provision of free condoms increased the condom usage rate from 31.73% in the pre-intervention phase to 60% in the postintervention phase.