While prison is recognised as a setting for infectious disease transmission among drug users, little is known about psychological and situational factors influencing high-risk behaviours, knowledge vital to prison-based interventions. Qualitative interview and focus group data were collected from staff and prisoners in six Scottish prisons. A general view was that prison heroin use had increased, but injecting and sharing remained a covert and minority behaviour. "Anti-injecting culture" among staff and most prisoners emerged as an important factor, though not linked by prisoners to an "anti-drug culture". Of individual and social risk factors identified, only the desire to inject in prison for maximum effect was unique to prison injectors and sharers. This decision-based behaviour requires further theory-focussed research. Given these findings, introducing needle exchanges into Scottish prisons could undermine their low drug injection rates. Enabling injecting, albeit within a public health framework, conflicts with the major prison objective of rehabilitation.