Patients with locally advanced (category T3-4 of the TNM system) and metastatic prostatic cancer, not previously treated, seen by one of the authors (P.H.S.) have been entered into EORTC Protocol 30762 which has compared the therapeutic effects of estramustine phosphate (Estracyt) and of diethyl-stilboestrol (DES) as primary treatment. A gradual and as yet unexplained rise in the total leucocyte count was seen in patients treated with estramustine phosphate. This was always apparent within 2 months of starting treatment and did not change significantly thereafter unless treatment was stopped, when the raised values soon returned to normal. In four patients in whom the white cell count rose to levels above the normal range a neutrophil leucocytosis was always reported. The cause of this is not yet understood.