Recent studies suggest even low dosages of oestrogen currently used for treatment of prostate cancer increase cardiovascular morbidity. In addition, relapse following growth of hormone - insensitive cells, and the present observations of further evidence of immunosuppression demonstrated by the significant (p less than 0.001) effect of DES on the lytic activity of natural killer cells vs. the negligible effect of the luteinizing - hormone - releasing - hormone, leuprolide (Lupron) raises concern that the palliative effects of oestrogen therapy are possibly further compromised by a reduction in immunosurveillance to tumour, or equally important, by a decreased capacity to cope with infectious agents.