A study by Wilson & Brewer (1992) has indicated that people with mental retardation are at greater risk of having a crime committed against them than age-matched cohorts from the general population. The present study was designed to test the hypothesis that this heightened vulnerability is partially explained in terms of behavioural shortcomings reflecting interpersonal competence. Twenty victims of crime and 20 nonvictims, all with mental retardation, were recruited so that means for age, adaptive behaviour and IQ were similar. Groups were compared on the Test of Interpersonal Competence and Personal Vulnerability (TICPV) developed for the present study. Results showed poorer interpersonal competence among the victims, indicating that victims had difficulty in deciding on the appropriate behaviour in some interactions. Performance on the test was shown to be internally consistent, stable over time and a valid measure of vulnerability. The study concluded with recommendations for more detailed investigation of the precise behaviours which influence risk as a first step in an attempt at remediation.