Nine hundred and nineteen injecting drug users (IDUs) were interviewed in Glasgow, Scotland during 1990 and 1991, as part of a wider study of HIV risk behaviour, about their injecting and sexual behaviour outside the city in the previous two years. Forty-five percent of respondents injected outside Glasgow, 6% shared needles and syringes (n/s) and 20% had sexual intercourse. Much activity occurred outside Scotland but mainly within the UK, particularly London. Predictors of n/s sharing outside Glasgow during the previous two years included current injecting with and passing on of used n/s and sexual intercourse with casual partners. Predictors of sexual behaviour outside Glasgow included passing on used n/s, having sexual intercourse with casual partners and, for females, engaging in prostitution. Glasgow IDUs are a highly mobile group and although HIV prevalence remains low within this population, considerable potential for importation/exportation of HIV and other bloodborne and sexually transmitted infections exists. Further work is required to establish why IDUs travel to, and engage in high-risk activities in locations outside their home environment, and detailed data about activities such as frequency of condom usage and n/s cleaning practices need to obtained. While there is a widespread network of services for IDUs in the UK, information provided usually relates to local services and may not fully address the needs of this mobile population. Therefore, we recommend that IDUs be provided with details of facilities such as n/s exchange schemes and drug-treatment establishments in centres to where they most commonly travel.