Studies of the prevalence of previously unrecognized physical illness among psychiatric patients have paid little attention to the treatment implications of such illness. The authors describe a study in California in which 78 inpatients received an augmented evaluation one to two weeks after their admission evaluation. The retest evaluation detected previously unrecognized physical conditions that were judged to be causal among patients and physical conditions that were judged to exacerbate the psychiatric condition among 56 patients. The authors discuss the treatment implications of the most common type of conditions detected, neurological and nutritional. They also delineate the barriers to recognizing and then treating previously unrecognized physical illnesses.