Background and purpose: Only limited data exist on the outcome of curative radiotherapy in patients who develop a second primary lung tumour after pneumonectomy. The treatment of eight such patients is described. Materials and methods: The case records of patients who underwent curative radiotherapy for stage I non-small cell lung cancer after a previous pneumonectomy were reviewed. Treatment was delivered using 3D external radiotherapy to a dose of 50-70 Gy, in once-daily fractions of 2-2.5 Gy. An endobronchial brachytherapy boost was used in three patients. Original treatments were re-planned in an attempt to minimize the volume of irradiated lung. Results: A complete remission was achieved in five (of six) evaluable patients, but two patients subsequently developed a local relapse. All patients survived for a minimum of 1 year after treatment. Only one patient developed significant (grade 2) radiation pneumonitis. When treatments were re-planned to optimize beam arrangements, and when customized blocks were used, the mean lung volume receiving ≥ 20 Gy (calculated for 70 Gy) decreased from 24.6 ± 4.1 (range, 18-31%) to 17.3 ± 5.1% (range, 12-26%). Similarly, the radiation conformity index improved from 0.44 ± 0.11 to 0.61 ± 0.06. Conclusions: Involved-field radiotherapy can be curative in patients who develop a new lung tumour after pneumonectomy. Recent advances in defining target volumes, treatment planning and delivery are likely to improve upon these results.