Compared with normal breast epithelium, increased caspase 3 immunopositivity was observed in 22% of benign epithelial hyperplasias, 25% of atypical hyperplasias, 58% of in situ carcinomas and 90% of invasive carcinomas. Staining was mainly diffuse and intracytoplasmic. The corresponding percentage expression for caspase 6 and 8 was 11%, 25%, 60%, 87% and 22%, 57%, 84%, 83%, respectively. As well as showing diffuse intracytoplasmic staining, caspase 6 and 8 also showed a granular staining pattern, the extent of which varied between tumours. With in situ lesions, immunoreactivty of all three caspases increased acccording to histological progression. The apoptotic index was reduced in benign and atypical hyperplasias compared to in situ or invasive carcinomas, but was not related to oestrogen or progesterone receptor expression. Overall, strong caspase 3, 6 and 8 immunopositivity was significantly associated with the extent of apoptosis.