A cohort of 6168 schizophrenic patients was followed from 1957 to 1984 to determine the incidence of cancer in these patients. In the male schizophrenic patients the incidence of cancer was found to be significantly reduced in comparison with the general Danish population. This reduction was especially marked for cancer in the respiratory system, cancer of the prostate and cancer of the bladder. In the female patients the overall incidence of cancer did not differ from that of the general Danish population, but there was an increased risk of cancer of the digestive tract, especially cancer of the pancreas and a slight increase of the risk of breast cancer. In the female patients the risk of respiratory cancers and cancer of the female genital organs, especially cancer of the uterine cervix, was reduced. These alterations of the incidence of cancer in schizophrenic patients cannot be ascribed to differences in diagnostic accuracy. As a possible explanation of these findings a reduced exposure to well known carcinogens such as cigarette smoke may be relevant. We speculate that exposure to neuroleptics such as phenothiazines and reserpine may also be part of the explanation for the findings.