A prospective study of the accuracy of patient positioning in mantle field irradiation was carried out in 13 lymphoma patients treated with curative radiotherapy. Patients were treated in the supine and prone position for anterior and posterior fields, respectively. Individually shaped divergent shielding blocks were placed in a fixed position in a template which was positioned on a tray above the patient. A total number of 94 megavoltage portal films (MV) was analysed and compared to 26 simulation films (SIM). MV-SIM differences were larger for posterior fields than for anterior fields. Regarding the position of the lung shielding blocks, mean MV-SIM differences ranged from 1.3 to 4.4 mm and errors exceeding 1 cm were found in 7.2% of cases. Most discrepancies appeared to be randomly distributed. A 4-5 mm systematic cranial shift of patients in the posterior treatment position was noted. Discrepancies in the position of the laryngeal block, spinal cord shielding block and humerus blocks were small with mean MV-SIM differences ranging from 0.3 to 2.7 mm. Differences between simulation set-up and treatment set-up were modest as compared to error rates reported in the literature. Shielding of tumour-bearing areas did not occur. It was concluded that the present standardised technique of patient positioning and the design of treatment fields results in acceptable error rates. Attention should be directed towards increasing the stability of patients in the prone treatment position in order to further reduce both systematic and random error rates.