The reproducibility of stereological estimates of mean nuclear volume by using the principle of volume estimation of particles of arbitrary shape is investigated together with the possible prognostic value of the estimates in the course of advanced prostatic cancer. Repeated transurethral resection of the prostate was performed 14-93 months after the first resection in 14 of 85 consecutive patients in whom prostatic cancer was newly diagnosed in the period 1979-1983. Twelve of these fourteen patients needed endocrine treatment because of progression of the metastatic disease. The reproducibility of the estimations was excellent as the variation of the method was less than 1% when the estimation of the mean nuclear volume was repeated at random on the same specimens after a time-interval of 2 months. The progression of the disease was evidenced by a significant histological upgrading and by a highly significant increase of the mean nuclear volume from the first to the second transurethral resection of the prostate.