Perfectionism can maintain depression, anxiety and eating disorders, yet few studies have evaluated treatments for perfectionism. This study examined the effectiveness of individual cognitive behaviour therapy (CBT) in treating perfectionism in four adults with a diagnosis of either an anxiety disorder or depression. The study used an A-B single case experimental design series with follow-up, and a 3-week pre- and postbaseline phase. Treatment involved 8 sessions and a 2-week follow-up session. Visual inspection of data revealed downward trends in overall perfectionism and clinically significant decreases in perfectionism for two participants. No clinically significant reductions were observed in depressive or anxious symptomatology. CBT for perfectionism warrants further investigation.