Neoadjuvant radiation or chemoradiation is currently the treatment of choice for patients with locally advanced carcinoma of the rectum. To assess the effects of chemoradiation on tumour regression and on uninvolved mesorectal lymph nodes, a consecutive series of 76 patients receiving neoadjuvant chemoradiation and a stage-adapted control series of 57 patients without pretreatment were studied. Densities of cells positive for CD4 (T-helper cells), CD8 (cytotoxic T-cells), CD83 (mature dendritic cells), and CD57 (natural killer cells) were determined on immunostains. Tumour regression was graded, and presence or absence of extramural tumour was recorded. The densities of CD4+ T-lymphocytes and CD83+ dendritic cells in the paracortex of mesorectal lymph nodes were observed to be significantly reduced, as were the densities of CD57+ cells in the follicles; densities of CD8+ T-lymphocytes did not differ. Strong, moderate and poor tumour regression was observed in 29, 25, and 22 cases, respectively. For 12 patients, absence of extramural vital or regressing tumour was recorded, indicating pretherapeutic overstaging. The results bring to mind that neoadjuvant chemoradiation as a side effect may have a negative impact on anti-tumour immunity. Together with the drawback of overstaging the results argue for a careful selection of patients.