Objective: Randomized controlled trials have attested the efficacy of cognitive behavioral therapy (CBT) in reducing psychotic symptoms. Now, studies are needed to investigate its effectiveness in routine clinical practice settings. Method: Eighty patients with schizophrenia spectrum disorders who were seeking outpatient treatment were randomized to a specialized cognitive behavioral intervention for psychosis (CBTp; n = 40) or a wait list (n = 40). The CBTp group was assessed at baseline, posttreatment, and 1-year follow-up. The wait list group was assessed at baseline, after a 4-month waiting period, at posttreatment, and after 1 year. The primary outcome measure was the Positive and Negative Syndrome Scale (PANSS). Results: The CBTp group showed significant improvement over the wait list group for the total PANSS score at posttreatment-postwaiting. CBTp was also superior to the wait list group in regard to the secondary outcomes positive symptoms, general psychopathology, depression, and functioning, but not in regard to negative symptoms. The number of dropouts during the treatment phases was low (11.3%). Participants perceived the treatment as helpful (98%) and considered themselves improved (92%). Significant pre- and posttreatment effect sizes varied between 0.77 for general psychopathology and 0.38 for delusional conviction. The positive effects of treatment could be maintained at 1-year follow-up, although the number of patients who had deteriorated was higher than at postassessment. Conclusions: Large proportions of patients in clinical practice settings benefit from CBTp. The efficacy of CBTp can be generalized to clinical practice despite the differences in patients, therapists, and deliverance. (Contains 2 figures and 4 tables.)