Psychiatrists' axis III diagnoses of physical illnesses in 357 psychiatric patients were compared with diagnoses by a physician's assistant using a standardized medical history form. The physician's assistant detected nearly three times as many physical illnesses as the psychiatrists. The psychiatrists were significantly more likely to miss diagnoses among older patients and women. Patients who met criteria for depressive disorders appeared to be at greatest risk for undetected illnesses, followed by patients with bipolar disorders and schizophrenia. Patients with a secondary diagnosis of substance abuse had significantly more undetected illness than those who did not abuse substances. The authors suggest that current axis III guidelines are inadequate and that a systematic review of physical health problems should be part of the psychiatric diagnostic assessment.