The potential mediating roles of danger expectancies, responsibility, perfectionism, anticipated anxiety and self-efficacy on obsessive-compulsive handwashing were examined. Ratings were obtained from 27 obsessive-compulsive patients before and during a behavioural avoidance test involving a compound stimulus of potting soil, animal hair, food scraps and raw meat. Correlation and partial correlation analyses were conducted to examine the relationships between the cognitive predictor variables and anxiety, urge to wash, time in task and duration of post-test washing. Large and significant relationships between danger expectancies and the dependent measures were obtained, even when alternative mediators were held constant. No other variable remained significantly related to any of the dependent measures when severity of disease ratings were held constant. Results suggest that of the cognitive variables examined, danger expectancies are the most likely mediator of washing-related behaviour in obsessive-compulsive disorder.