Capacity to consent is one of the linchpins of the ethical conduct of clinical care and research, and it needs to be reliably measured. The authors describe the development of a new measure of the "appreciation" component of capacity, the California Scale of Appreciation (CSA), 18 items rated according to the concept of "patently false belief" (a belief that is grossly improbable); 39 patients with schizophrenia or a related psychotic disorder (27 outpatients and 12 inpatients) and 15 normal-comparison subjects participated. Each subject's audiotaped interview was rated by three evaluators. Answers to each item were scored as "capable," "incapable," or "uncertain capacity." Also, each subject was given an overall rating of one of these three categories by each rater. Total scores on the CSA were calculated and correlated with scores on standardized instruments for assessing psychopathology and cognitive impairment. The mean total CSA score was significantly lower in the patients than in the normal-comparison subjects; however, a majority of the patients were found to be fully "capable" on the CSA. The CSA is a potentially useful instrument for measuring the appreciation component of capacity in persons with psychotic disorders. Its generalizability to other patient populations and to other types of protocols needs to be determined.