Lymphopaenia is the earliest and the most sensitive routinely assessed biological parameter of corporeal radiation exposure in clinical practice; bone marrow, lymph nodes and peripheral blood lymphocyte populations are also at risk. During radical prostate radiotherapy, in 28 patients, the mean peripheral lymphocyte count fell from 1.76 × 109/l (standard deviation (SD) 0.63, 95% confidence interval (conf.) 0.23) to 1.10 × 109/l (SD 0.38, conf. 0.14), (p < 0.05). The question was asked as to whether intensity-modulated radiation therapy (IMRT) by TomoTherapy would cause more lymphopaenia than three-field conformal radiotherapy, bearing in mind the ‘low dose bath’ effect of IMRT and the long ‘beam-on’ times. Thirteen patients receiving three-field conformal radiotherapy experienced a fall in peripheral lymphocyte counts from 2.02 (SD: 0.62. conf. 0.43) to 1.17 × 109/l (SD: 0.47, conf. 0.26) after 34–38 Gy, as compared to a fall from 1.6 × 109/l (SD: 0.6, conf. 0.35) to 1.04 × 109/l (SD: 0.3, conf. 0.15) for 15 TomoTherapy patients—non-significant differences. We conclude that for this (approximately) standard, small-volume pelvic radiotherapy and to the dose under scrutiny, we cannot detect differences between the two radiotherapy techniques in terms of the lymphopaenia accruing. Neutrophil counts were similarly non-significantly different.