Thirty-five patients with strongly suspected recurrent tumor of the lung and a definitely positive CT scan were reviewed. The patients had undergone surgery (group A, n = 17) or radiation therapy (group B, n = 18). TNM staging of lung cancer in both groups revealed similar results. Small-cell carcinoma (p less than 0.05), central tumors (p less than 0.003) and elder patients (p less than 0.05) were more often found in group B. The disease-free interval was longer in patients with tumor resection (45.5 vs 11.7 months, p less than 0.007) and depended on the T-stage in irradiated cases (p less than 0.05). Local recurrence with or without mediastinal lymph node involvement occurred in all irradiated patients: 3 out of 17 surgical patients showed isolated mediastinal lymph node enlargement without tumor relapse (not seen by plain chest roentgenographs). Plain films failed to detect nearly 20% of space-occupying lesions, which could easily be identified by CT. In one patient the suspected tumor recurrence turned out to be a tuberculous infiltration. A second lung cancer--no tumor recurrence--was pathohistologically assumed in 3 of the resected cases with an interval of from 10 to 181 months after surgery. On the basis of these findings, CT monitoring can be recommended when the patient is resected for cure. Some patients will benefit by an early diagnosis of local regional tumor recurrence when the time until the necessary secondary treatment can be shortened. Long-term-survival may be achieved in a small part of these patients.