From November 1957 to June 1984, 30 patients with multiple primary lung cancer were diagnosed, basing on clinical features, diagnostic means, histologic type, treatment and prognosis. Out of 3,815 cases of resected primary lung cancer, the incidence of multiple primary cancer was 0.8%. There were 10 synchronous and 20 metachronous cases. There were 17 unilateral, only 1 simultaneous bilateral and 12 contralateral after resection of cancer in the opposite lung. Four of the 10 synchronous cases were definitely diagnosed preoperatively. Among the 20 metachronous cases, 9 were definitely diagnosed as a second primary lesion and the other 11 were proved by thoracotomy. Pathologically, 19 had identical types (15 squamous cell and 4 adenocarcinoma) but 11 had double types (9 squamous cell + adenocarcinoma, 2 squamous cell + anaplastic). The average survival of these 30 patients was 27.1 months, that in the synchronous group was 29 months and that in the metachronous group was 26.2 months. The 5 year survival rate of the synchronous cases was 35%, that of the metachronous cases was 42%. The clinicopathological criteria of multiple primary lung cancer, early diagnostic and operative procedure are also discussed.