One hundred cancer patients undergoing active treatment were interviewed to determine how they perceived their illness and how their perceptions compared with those of their attending physicians. Ninety-eight patients recognized that they had cancer and 87 correctly identified the tumour type. Sixty-four of 67 patients with local or regional disease were aware of this, but 11 of 33 patients with metastatic disease incorrectly believed that the cancer was localized. Five of 52 patients being treated for cure thought they were being treated palliatively, and 16 of 48 patients receiving palliative treatment believed that the doctor's aim was to cure them. Forty of these 48 patients significantly overestimated the probability that the treatment would prolong their lives. Patients with little secondary education were significantly more likely to underestimate the seriousness of their condition. Interactions between doctor and patients were not observed directly and it was therefore not possible to determine whether patients' inaccurate views of their illness were due to suboptimal communication or denial. Doctors frequently failed to recognize their patients' misconceptions. In only one of the 16 cases in which a patient, who was being treated palliatively, believed that the treatment was curative did the doctor recognize that this misunderstanding existed.