Background: This was a quantitative study to measure the prevalence low self-esteem in an intellectually disabled forensic population. The dependent variables used were the adapted six-item Rosenberg Self-Esteem Scale and the adapted Evaluative Beliefs Scale. It had a repeated measures design with independent variables including consideration of differences between the low and medium secure parts of the service, the influences of types of offences and the effects of disrupted childhood attachments. Methods: Forty-four male clients, with mild to borderline intellectual disabilities, were recruited. Data were collected by one key researcher during individual research interviews using the two structured instruments to measure self-esteem. Further data were then obtained from routinely recorded clinical information held on the hospital computer system. Results: Unexpectedly, the majority of clients scored as having moderate or high self-esteem on both self-esteem measures. There was a statistically significant positive correlation between the two scales. In all, 64% of the population studied had committed either sexual offences or fire setting offences. Contrary to expectation, those who had evidence of disrupted attachments had slightly higher self-esteem than those who had not experienced disrupted attachments. Conclusions: Self-esteem is a complex personal concept with many influencing factors. Cognitive behaviour therapy has a unique role in realising and overcoming negative core beliefs and feelings of low self-worth. The offence types concurred with previously noted patterns of offending within the intellectually disabled forensic population.