Apoptosis is the genetically mediated mechanism by which individual cells are deleted from living tissues. The extent of apoptosis in human tumours has not been examined in detail. In this study, apoptotic indices were assessed semiquantitatively by light microscopy in routine haematoxylin and eosin (H & E) stained sections of 15 high-grade carcinomas of the prostate by light microscopy, using the 40 x objective, counting 2000-2500 tumour cells per case. The mean apoptotic index for nine solid undifferentiated carcinomas was 1.7% (range 0.2-2.4%), for three small cell undifferentiated carcinomas 25.2% (range 10-37%), and for three secondary transitional cell carcinomas of the prostate 5.5% (range 2.4-9.1%). In general, apoptosis was easily identified, using the 40 x objective. There were minor variations in apoptosis from field to field in each tumour, and greater variability in the apoptotic index was observed among the small cell undifferentiated carcinomas and transitional cell carcinomas. In conclusion, assessment of apoptosis in routine H & E sections of human tumours is feasible, but it remains to be determined whether the extent of apoptosis correlates with other determinants of biological behaviour.