Patient and tumour characteristics of 23 patients presenting with a second primary lung cancer were analysed and compared with 534 patients with radically resected stage 1 non-small cell lung cancer (NSCLC). None of these characteristics is associated with a higher occurrence rate for second primary lung cancer. Prognosis in the latter patients is significantly worse than after resection of a 'solitary' NSCLC: the median survival time (MST) after resection of the first tumour is 50 months; after diagnosis of the second tumour only 14 months. Surgically retreated patients have a prognosis that is similar to that after resection of a 'solitary' NSCLC. No separate independent prognostic factors responsible for this survival difference could be isolated. Squamous histology and central location are associated with a longer recurrence free survival time. We conclude that the occurrence of a second primary lung cancer can not be predicted based on patient or tumour characteristics and that only surgical retreatment offers a chance of long survival in these patients.