This study was done to investigate the frequency of co-morbidity and to demonstrate the best method for assessing depression among cancer patients. The subjects were 50 (25 male and 25 female) cancer patients and 50 (25 male and 25 female) medically ill patients. All subjects were interviewed by psychiatrists and were administered psychological tests such as SAS (self-rating anxiety scale), SDS (self-rating depression scale), POMS (Profile of Mood States), HADS (Hospital Anxiety and Depression Scale) and DRP (Depression-related personality traits). The psychiatric interview revealed that 44% of cancer patients and 38% of the medical patients had mental disorders according to DSM-IV. The most frequently observed disorder was depression, which was seen in 28% of the cancer patients and 30% of the medical patients. The cancer patients with depression scored significantly higher on the DRP and the Anger mood state of POMS than did the medically ill patients with depression. In addition, most psychological tests employed had no discrimination between depressed and normal subjects among the cancer and the medical patients. However, it was found that the Depression scale in HADS (HADS-D) split depressed patients from normal subjects since the HADS-D was composed of items that were not concerned with physically ill conditions.