Thirteen cases of double primary cancer, arising in the lung and the other site including the stomach (7 cases), colon (3 cases), esophagus (1 case), uterus (1 case) or breast (1 case), were experienced in our department during the recent one decade from January 1979 to April 1989. These cases accounted for 7.0% of all cases of lung cancer treated during the same period. The digestive tract was predominant as the other site for both synchronous and metachronous double primary cancer. There was no significant difference in age, sex, or histological type. The 5-year-survival rate was 17.0% in 12 cases in which operation was performed for lung cancer. Of these, death occurred in 6 cases. Two deaths were caused by cancers other than lung cancer, and one by a complication after surgery for a cancer other than lung cancer. The 5-year-survival rate for the remaining 9 cases was 28.2%. In the treatment and follow-up of lung cancer, more attention should be paid to the possibility of a secondary primary cancer, especially in the digestive tract. Earlier diagnosis and aggressive treatment may improve the survival rate.