Four years after the occurrence of an outbreak of hepatitis B and HIV infection among injecting drug user inmates at Her Majesty's Prison Glenochil in Scotland, a study design was developed to complete the epidemiological account of the HIV outbreak. Our aim was to identify potential cases of (1) HIV transmission not diagnosed during the original outbreak investigation and (2) the source(s) of the outbreak. Scotland's HIV positive case register was searched for matches to a soundexed list of 636 Glenochil inmates imprisoned during January-June 1993. Eight HIV infections that may have been acquired in Glenochil and four possible sources of the outbreak were identified. The second stage of follow-up molecular epidemiological techniques used on stored sera samples from identified individuals is described in the companion paper. Without breach of medical or prisoner confidentiality, indirect and anonymous follow-up has proved possible for the Glenochil inmates.