Relatively little is known about the efficacy of clinically inexperienced student therapists carrying out cognitive behavior therapy (CBT) under supervision during a professional, psychologist training program. The current study evaluated this by collecting pre- and post-treatment data on 591 consecutive patients receiving treatment at the Psychotherapy Clinic of the Department of Psychology, Stockholm University, Sweden, over an 8-year period. The patients had mainly anxiety disorders or depression with a mean duration of 15 years, and received individual CBT for a mean of 18 sessions. They improved significantly on both general measures (Beck Anxiety Inventory [BAI], Beck Depression Inventory [BDI], and Quality of Life Inventory [QOLI]) and disorder-specific self-report scales. The proportions of recovered patients on the BAI (63%) and the BDI (60%) were higher than those of a comparison effectiveness study. On the specific self-report scales the current sample improved as much as the samples in extant efficacy trials. We conclude that clinically inexperienced student therapists who receive supervision from experienced supervisors can achieve treatment effects that are on a par with those of experienced licensed psychotherapists. (Contains 7 tables.)