Among 337 cases of surgically resected lung cancer from April 1977 to March 1984, there were 24 cases (7.12%) of multiple primary cancers including 22 cases of double cancer and one case each of triple and quadruple cancer. They consisted of 17 males and 7 females and the patient age at the time of the lung resection ranged from 32 to 81 years old (average: 67.4). In double cancer cases, the most common site of the other primary cancer was the lung (9 cases), followed by the stomach (5 cases), the colon (4 cases), the pancreas (2 cases), and there was one case each of the salivary gland and the skin. The cases of triple and quadruple cancer involved multiple primary lung cancer associated with mammary and esophageal cancer, respectively. There were 14 synchronous and 8 metachronous cases. The histology of lung cancer in 13 cases of double cancer was adenocarcinoma in 6, squamous cell carcinoma in 2, large cell carcinoma in 3, small cell carcinoma in 1, and adenosquamous cell carcinoma in 1. The histology of 11 cases of double or triple primary lung cancers was squamous cell carcinoma in 17, adenocarcinoma in 5, and large cell carcinoma in 1. In 24 cases 19 lobectomies, 4 pneumonectomies and one partial resection of the lung was performed for the first primary lung cancer. All cancer lesions of other organs apart from the lung were also surgically resected.(ABSTRACT TRUNCATED AT 250 WORDS)