The hypothesis that Asperger's syndrome (AS) may go unrecognised in forensic populations was examined by ascertaining the prevalence in Broadmoor Special Hospital. The entire male patient population was screened by examination of case notes. Identified cases were subject to the next stage of the study, which involved observation and interviewing of patients, and a semi-structured interview of key staff. A prevalence of 1.5% (0.6% to 3.3%, 95% CI) was found. The addition of equivocal cases increased the prevalence to 2.3%. The prevalence of AS in Broadmoor Hospital is greater than that reported for the general population.