Twenty-four patients receiving post-operative radiotherapy for testicular tumours have been studied to assess the impact of treatment errors. Regular treatment verification radiographs were performed and compared with simulator planning radiographs. It was demonstrated that lymph nodes in the area most at risk, i.e., para-aortic nodes at L1 to L3 on the ipsilateral side, were inadequately irradiated on 10% of treatments, and that at least one lymph node miss was recorded in 12 of the 24 patients. These results are considered to be a minimum estimate. Mostly para-aortic fields 8.0 cm wide were used, but it was shown that the number of lymph-node misses could have been substantially reduced by using fields 9.0 cm wide, whilst the consequent increase in irradiated kidney volume was considered acceptable.