We carried out a study to evaluate treatment response and residual mass in lung cancer with positron emission tomography (PET), using l-[methyl-¹¹C]methionine (MET). MET tumour uptake and tumour volume measured by computed tomography (CT) before and within 2 weeks after radiotherapy or chemoradiotherapy were compared in 43 studies of 21 patients. Ten patients with local control (no recurrence) of tumour showed a larger decrease in MET uptake (65.2%±12.2%) than in tumour volume (50.8%±9.6%, P<0.01). Five patients with early recurrence (from 1 to 4 months) showed smaller decreases in both MET uptake (22.2%±13.5%) and tumour volume (28.6%±20.0%) than those in the no-recurrence group (P<0.01). Four patients with late recurrence (after 11 months or more) showed a similar decrease to the no-recurrence group in MET uptake (72.8%±14.8%) but little change in tumour volume (18.5%±19.0%), the latter result corresponding to that in the early-recurrence group. Using tumour volume only, the no-recurrence group was differentiated from both the early- and the late-recurrence group (P < 0.01), but the early-recurrence group was not differentiated from the late-recurrence group. Using the MET uptake data, the early-recurrence group was clearly distinguished from the late-recurrence group (P<0.01), but the late-recurrence group was indistinguishable from the no-recurrence group. CT was useful in distinguishing the no-recurrence group from the groups in which there was ultimate recurrence, whether early or late. When a residual mass is seen on CT, PET seems to be helpful in evaluating tumour viability.