The incidence of multiple primary malignancies has increased in recent decades. The present study attempts to determine the clinical characteristics, the smoking factor, prognosis and temporal relationship of lung cancer to other cancers in patients with multiple primary malignancies. A total of 193 patients with multiple primary cancers involving lung cancer were found among 22,405 cancer cases diagnosed in Taipei Veterans General Hospital, between 1993 and 1997. Patients' clinical characteristics, smoking habit, tumor location, lung cancer histology, staging and survival were recorded and analyzed. The results showed that smoking is a significant risk factor for the development of multiple primary malignancies involving lung cancer (P<0.001). Of the 193 patients in this study, 51 had lung cancer diagnosed before the occurrence of other primary cancers (lung cancer first group, LCF group) and the remaining 142 patients had another cancer site develop ahead of the lung cancer (other cancer first group, OCF group). There was a significant difference between the time of the diagnosis of the first primary cancer to that of the second primary cancer in the LCF group and in the OCF group (median 10 vs. 46 months, P<0.001). For lung cancer staging, 53.3% of LCF patients suffered from stage I-II lung cancer, while 24.5% of OCF patients suffered from stage I-II lung cancer. Upper aerodigestive tract tumors were the most frequent tumors accompanying lung cancer, followed by colorectal and cervical cancer. Patients with cervical cancer were at a higher risk of developing lung cancer. Median survival was 65 months in the LCF patients and 81 months in the OCF patients, when calculated from the diagnosis of the first cancer (P=0.558). Median survival was 36 and 14 months, respectively, when calculated from the diagnosis of the second cancer (P=0.081). Median survival (37 vs. 14 months, P=0.085) and 3-year survival (62.5 vs. 25.4%, P=0.002), calculated from the diagnosis of the second primary lung cancer, was better in those LCF patients who developed another primary lung cancer than in the OCF patients who developed a second primary lung cancer. In conclusion, smoking is a risk factor for the development of multiple primary cancers. Upper aerodigestive tract cancer, colorectal cancer and cervical cancer were the tumors most frequently accompanying lung cancer. The staging status and median survival of patients who had a second primary lung cancer were better than in the general lung cancer population. Careful follow-up and intensive treatment is suggested for these patients.