I found these interesting figures on the net
Johnson (1968) performed detailed physical exams on 250 patients admitted to an inpatient psychiatric unit. 12% of these patients were admitted to the psychiatric unit for problems that seemed to be caused by physical illness
80% of these had been missed by physician before admission
6.6% were initially missed even after the admission workup
60% had abnormal physical findings
Ex.-Hall (1978) performed a detailed assessment on 658 consecutive psychiatric outpatients - 9.1% had a significant medical illness-
Ex.-Slater (1965) studied 85 patients (32 men and 53 women) diagnosed as having "hysteria" - follow up 7-11 years. More than a third proved to have organic disease
Ex.-Sox et. al. (1989) did a thorough medical evaluation on 509 patients in community mental health programs in California.- 200 (or 39%) had at least one active, important, physical disease, Staff at the mental health program was aware of only 47% of these. Research program discovered previously undiagnosed, important diseases in 63 of these patients. 14% had medical illness that was causing or exacerbating their mental illness.
Source: www.alternativementalhealth.com/articles/diamond.htm