To determine which factors impact on the efficacy of cognitive behavioural therapy (CBT) for depression and anxiety. Factors considered include those related to clinical practice: disorder, treatment type, duration and intensity of treatment, mode of therapy, type and training of therapist and severity of patients. Factors related to the conduct of the trial were also considered, including: year of study, country of study, type of control group, language, number of patients and percentage of dropouts from the trial.