Plan and conduct of the studyThe plan and conduct of the study were described in detail previously (Medical Research Council, 1971). In summary, after resection of all intrathoracic bronchial tumour, the patients were allocated at random to receive tablets of busulphan (B series), cyclophosphamide (C series), or indistinguishable placebos (P series) daily for 2 years. For the first 10 days following operation, all patients received 8 tablets in a single daily dose (B series 4 mg, C series 200 mg). Thereafter they received 6 tablets daily (B series 3 mg, C series 150 mg). However, after about the first year of intake to the study, these maintenance dosages were reduced from 6 to 3 tablets (B series 1.5 mg, C series 75 mg), because of an unexpectedly high incidence of toxicity. The 3 series were very similar with respect to sex, age, type of operation, bronchial site of tumour, histological type of tumour, and whether or not the resected lymphnodes were histologically involved.Each patient was reported on monthly during the first 3 years, 3-monthly up to 5 years, and annually thereafter. Total leucocyte and platelet counts and haemoglobin concentrations were measured every month during the first 2 years, and thereafter when requested by the clinician. The maintenance dose of tablets was controlled by the clinician from the results of the blood investigations and the dose was reduced if haematological toxicity was suspected.The study was conducted double-blind. The certified cause of death was obtained from the local centre and was verified from the death entries at the OPCS for all but 2 of the deaths.It is important to note that as the study was planned in 1964, some of the peri-operative staging procedures which would now be considered desirable were not required as a routine and when done were not necessarily reported. In particular, the following were not specifically requested as part of the protocol: recording recent weight loss, mediastinoscopy, isotope scans, liver function tests, marrow aspiration or biopsy, routine dissection of the mediastinal nodes at operation, or reporting the size of the tumour in the resected specimen.X3