Cases of multiple primary cancer (MPC) with gastric cancer were collected from 66 hospitals by questionnaires sent to 114 hospitals and institutions in Japan. Four hundred forty-five out of 22,163 gastric cancer patients who were operated on between 1966 and 1970 and had at least 10-year follow-up data were registered as having more than one cancer. The overall frequency was 2.0%. In 137 patients the MPCs were synchronous in 125 the MPC appeared after gastrectomy and 186 patients had a previous cancer before the diagnosis of gastric cancer. The most frequent association was cancer in the alimentary tract (53%), followed by uterus, breast and others in that order. In the patients with gastric cancer, the relative risk of a second cancer seemed to be twice as high as the incidence of cancer in the general population, especially in alimentary organs (four times as high) and in thyroid (nine times). Second malignancies were the first cause of death, except for recurrence or metastasis of the initial gastric cancer, and the importance of a follow-up system for cancer patients was indicated.