The role of post-operative radiotherapy for patients with non-small-cell lung cancer (NSCLC) is unclear despite five previous randomised trials. One deficiency with these trials was that they did not include adequate TNM staging, and so the present randomised trial was designed to compare surgery alone (S) with surgery plus post-operative radiotherapy (SR) in patients with pathologically staged T1-2, N1-2. M0 NSCLC. Between July 1986 and October 1993, 308 patients (154 S, 154 SR) were entered from 16 centres in the UK. The median age of the patients was 62 years, 74% were male, > 85% had normal or near normal levels of general condition, activity and breathlessness, 68% had squamous carcinoma, 52% had had a pneumonectomy, 63% had N1 disease and 37% N2 disease. SR patients received 40 Gy in 15 fractions starting 4-6 weeks post-operatively. Overall there was no advantage to either group in terms of survival, although definite local recurrence and bony metastases appeared less frequently and later in the SR group. In a subgroup analysis, in the N1 group no differences between the treatment groups were seen, but in the N2 group SR patients appeared to gain a one month survival advantage, delayed time to local recurrence and time to appearance of the bone metastases. There is, therefore, no clear indication for post-operative radiotherapy in N1 disease, but the question remains unresolved in N2 disease.