Three clients with obsessive‐compulsive disorder were studied using a multiple case study approach. Clients' experiences of cognitive‐behavioural therapy, in particular the impact of having their beliefs challenged, and their responses to self‐monitoring tasks and behavioural experiments were investigated. Data included case notes made during therapy, pre‐ and post‐therapy questionnaires, and post‐therapy semi‐structured interviews. Qualitative analysis of the data identified clients' obsessive beliefs, general negative beliefs and meta‐cognitive beliefs, and explored any post‐therapy changes in these beliefs. The results demonstrated similarities between cases in obsessive beliefs (e.g. inflated responsibility for harm), general negative beliefs (e.g. feelings of failure), meta‐cognitive beliefs (e.g. the need to control thoughts) and affect (e.g. guilt). Questionnaire data from all three clients suggested no clinically significant post‐therapy changes in general negative beliefs and meta‐cognitive beliefs. However, idiosyncratic changes in obsessive beliefs were reported during therapy and at post‐therapy interview, and clients reported improved functioning and decreased levels of distress during therapy and at interview. Responses to challenge were uniformly positive, but reactions to self‐monitoring and behavioural experiments varied across cases. The implications of these findings for the treatment of obsessive‐compulsive disorder using cognitive behavioural techniques are discussed.