From November 1957 to June 1984 at Shanghai Chest Hospital, 30 cases of multiple primary lung cancers were confirmed, based on clinical characteristics, diagnostic process, histologic type, treatment, and prognosis. Out of 3,815 cases of resected primary lung cancer, the incidence of multiple primary lung cancers was 0.8 percent. There were ten synchronous cases and 20 metachronous cases. Seventeen cases were unilateral, and 13 cases were bilateral, of which only one case was synchronous, and the remaining 12 cases were postoperative resection of an opposite lesion. Among the ten synchronous cases, four cases of multiple primary lung cancers were definitely diagnosed before surgery by chest x-ray films or fiberoptic bronchoscopy. Among the 20 metachronous cases, 11 cases were definitely diagnosed before surgery as the second primary lesion by chest x-ray films taken during periodic follow-ups after the initial resection, while nine cases were proven by thoracotomy. All of the 15 cases definitely diagnosed before surgery as multiple primary lung cancers were according to our criteria. Histologically, adenocarcinoma was relatively scarce, at a rate of 13 percent (4/30); but epidermoid carcinoma was predominant, at a rate of 87 percent (26/30), of which 11 cases were accompanied by adenocarcinoma or large-cell undifferentiated carcinoma. The average postoperative survival in the ten synchronous cases was 29 months and in the 20 metachronous cases was 26.2 months, counting from the time of the second operation. The criteria of clinicopathologic findings, early diagnostic procedure, and surgery for multiple primary lung cancers were also discussed.