Tumour volume was related to metastasis, seminal vesicle invasion, capsule invasion, and histological differentiation in a series of 100 unselected prostates with carcinoma removed at necropsy and 38 removed at radical prostatectomy. All these variables were highly inter-related. In both series, metastases were associated only with tumours larger than 4 ml, a volume attained by only 13% of the necropsy tumours. Loss of differentiation was strongly correlated with tumour volume, and only tumours of Gleason grade 4 or 5 had metastasised. It was concluded that the natural history of prostate cancer is highly predictable. The capacity to metastasise probably develops only in tumours which have grown much larger than 1 ml and acquired poorly differentiated areas as a manifestation of the phenomenon of tumour progression. The unusually low proportion of metastatic prostate cancer is readily explained by the large proportion of small-volume tumours in this organ.