A total of 805 patients underwent lung resection for non-small-cell lung carcinoma at the University of Munich Medical Center, Klinikum Grosshadern, from 1978 through 1988. Microscopic residual disease at the bronchial margin was found in 21 patients (2.6%). The tumor residues showed either a mucosal (1%) or a extramucosal (1.6%) spreading pattern. Patients with extramucosal microscopic residual disease had a poorer prognosis (median survival 10.3 months) than patients with mucosal microscopic residual disease (median survival 25 months). The prognosis was better if the tumor was squamous cell as opposed to adenocarcinoma or large-cell carcinoma. The most important prognostic factor was tumor stage. Patients with microscopic tumor infiltration and stage I or II disease survived longer than the comparable stage III group. We suggest that these patients should undergo reoperation, if possible. Patients with stage III disease, mediastinal lymph node involvement, and microscopic residual disease have the same marked reduction in survival as patients with stage III disease but without microscopic tumor infiltration. We do not recommend a follow-up operation in these patients. Complete histologic examination of mucosal and extramucosal peribronchial tissues at the resection line by frozen section is mandatory to avoid leaving microscopic tumor behind, which may adversely affect patient survival.