BACKGROUND: The implementation of clinical guidelines in care pathways is being promoted for quality assurance in psychiatry and psychotherapy, as in other medical fields. The achievable benefits are disputed and are generally thought to be small. There have been hardly any studies of the effect of clinical care pathways on the costly inpatient treatment of schizophrenic psychoses. METHODS: We conducted a prospective, controlled, before and after study in 114 patients with schizophrenia to determine whether the implementation of a pathway would improve diagnosis and treatment in conformity with published guidelines, and whether there would be any associated improvement in outcome. The patients' course was extensively documented with a number of structural, process-related, and outcome-related variables in the years before and after pathway implementation. Moreover, two different intensive methods of pathway implementation were tested. Data were collected from 2003 to 2005. The primary indicators of outcome quality included pharmacotherapy-related variables and assessments of treatment efficacy by the physicians, the nurses, and the patients themselves. RESULTS: After pathway implementation, some diagnostic tests that had been performed only rarely beforehand were performed much more often. The percentage of over- or undermedicated patients, as defined by the treatment pathway, declined markedly. Surprisingly, however, the patients' multidimensionally documented psychopathological course and their subjective judgments of their condition were worse after pathway implementation than before on all four scales that were used to assess these variables. CONCLUSION: The implementation of a treatment pathway brought about a robust change in process-related variables. The findings of this study furnish no explanation for the observed decline in treatment efficacy.