Five hundred three injecting drug users in Glasgow recruited by a multisite and citywide sampling strategy were questioned regarding their drug-taking behaviour during episodes of custody over the six months prior to interview. Fifty-two percent had been in custody during the past 6 months, 16% of these had injected while in custody. Of these 73% borrowed injecting equipment and 78% handed on used equipment to others. All those who shared, cleaned their injecting sets before use. Over half of those who injected had a source of new sets. While the potential exists for spread of HIV among drug users while in custody there is clear understanding among them of the route by which the virus is spread and also the will to prevent it.