We evaluated the occurrence and type of malignant tumors in 148 patients with sarcoidosis followed at the Okayama University Hospital. Nine patients had malignancies; in 2 of 9 patients the development of malignancy preceded that of sarcoidosis, and one patient presented with sarcoidosis and malignancy at the same time. Six patients developed six types of malignancy following the development sarcoidosis; one case each of stomach cancer, lung cancer, breast cancer, thyroid cancer, testicular tumor, laryngeal cancer, and chronic lymphocytic leukemia. There was no significant difference between sexes (3 males and 3 females). The mean age of the cancer group at the onset of sarcoidosis was 56 years, which was significantly higher (p less than 0.05) than that of the control group. In these 6 patients, the mean interval from onset of sarcoidosis to detection of cancer was 11.7 years (range 1.5 to 30.2 years). The relative risk of malignancy was calculated based on the data for 148 patients with sarcoidosis with a total of 1371 person-years. The expected incidences of cancer for all sites and specific sites were estimated by applying age- and sex-adjusted person-years. The observed incidence of cancer was significantly (p less than 0.05) greater than the expected incidence for thyroid cancer, laryngeal cancer, and leukemia. No significant difference in incidence was found for all sites or for the other sites of cancer. The increased cancer incidence in sarcoidosis may be secondary to immunological abnormalities associated with this disease.