In an exploratory study of HIV risk behaviour and custodial experiences, 183 injecting drug users were interviewed in twelve cities in England, Scotland and Wales. Custodial experience was common (79% in custody at some time), recent (58% in custody since 1987), and sentences were short (for 64% the most recent period of custody lasted one month or less). Injecting during last period in custody was reported by 23%, and 75% of those who injected in custody reported that they had shred needles and syringes. Sexual activity in custody was reported by 6% of the custodial sample. HIV positivity was self‐reported by 12% of the custodial sample. Of the custodial group, those who were HIV positive were more like& than the HIV negative group to report injecting and syringe‐sharing in custody. Outside custody fin the community) many (46%) had shared syringes during the previous four weeks, 83% had sexual partners during the previous three months, and 50% of these had sexual partners who did not themselves inject drugs. The findings indicate the potential for HIV infection to occur within custodial settings.