OBJECTIVE The authors reviewed research on medication compliance in psychiatric treatment and compared compliance rates with compliance rates in treatment of physical disorders. METHODS MEDLINE was used to locate reports in the literature on medication compliance in psychiatric treatment for the years 1975 through 1996. These reports and studies cited in the reports were reviewed to determine the methods used to assess compliance and the compliance rates reported. Ten reports describing assessment methods and including medication compliance rates for antidepressant medication and 24 reports for antipsychotic medication were selected. They were compared with 12 reports that used microelectronic monitoring to assess medication compliance of patients with a range of nonpsychiatric disorders. RESULTS Studies of psychiatric patients used various methods of estimating medication compliance, including interviews with patients, clinicians' judgment, and pill counts, but overall showed low rates of compliance. Patients receiving antipsychotics took an average of 58 percent of the recommended amount of the medications, with a range from 24 to 90 percent. Patients receiving antidepressants took 65 percent of the recommended amount, with a range from 40 to 90 percent. The mean compliance rate for patients with physical disorders was 76 percent, with a range from 60 to 92 percent, although the microelectronic monitoring showed frequent omission of doses and discontinuation of medication. CONCLUSIONS Compliance with medication regimens among patients with psychiatric disorders may be lower than among patients with physical disorders. However, the difference may be largely attributable to the methods used for estimating compliance. The findings suggest the need for new and improved methods for monitoring compliance and increasing patients' compliance with pharmacotherapy.