We have carried out a detailed analysis of the cellular immune functions of breast cancer patients in comparison with healthy controls. A possible correlation between immune and clinical parameters was analysed in 50 breast cancer patients. Immune parameters, natural killer cell and T lymphocyte functions and the numbers of circulating T lymphocytes were analysed against the clinical parameters comprising the turnout burden, the stage of the disease and the expression of hormone receptors on the tumour. In order to analyse the immune function data effectively, low responders were identified with stringent cut-off values. Considerably higher proportions of low responders were found among the patient population. Elevated numbers of circulating T lymphocytes and CD3-directed cytolysis correlated with the expression of oestrogen receptors independently of the clinical/histological parameters.