Two cases of lung cancer were studied with positron emission tomography (PET) using L-[methyl-C-11]methionine (C-11 Met) and CT scans five to six times during long-term follow-up after radiotherapy. In a large cell carcinoma with mediastinal invasion, C-11 Met tumor uptake showed a rapid decrease after radiotherapy, corresponding to clinical improvement, and detected recurrence at 11 months, as confirmed by biopsy. Tumor volume by CT showed no significant changes during this time. A squamous cell carcinoma of the superior sulcus (Pancoast type) showed rapid changes in C-11 Met tumor uptake and similar changes in tumor volume during two courses of radiotherapy and recurrence over a period of 25 months. PET evaluation of tumor viability seems to be valuable for treatment evaluation, and results match the tumor volume changes measured by CT.