The protein kinase C (PKC) family of genes encode serine/threonine kinases that regulate proliferation, apoptosis, cell survival and migration. Multiple isoforms of PKC have been described, one of which is PKC delta. Currently, it is unclear whether PKC delta is involved in promoting or inhibiting cancer formation/progression. The aim of this study was therefore to investigate the expression of PKC delta in human breast cancer and relate its levels to multiple parameters of tumour progression. Protein kinase C delta expression at the mRNA level was measured using real-time PCR (n=208) and at protein level by both immunoblotting (n=94) and ELISA (n=98). Following immunoblotting, two proteins were identified, migrating with molecular masses of 78 and 160 kDa. The 78 kDa protein is likely to be the mature form of PKC delta but the identity of the 160 kDa form is unknown. Levels of both these proteins correlated weakly but significantly with PKC delta concentrations determined by ELISA (for the 78kDa form, r=0.444, P < 0.005, n=91 and for the 160 kDa form, r=0.237, P=0.023, n=91) and with PKC delta mRNA levels ( for the 78 kDa form, r=0.351, P=0.001, n=94 and for the 160 kDa form, r=0.216, P=0.037, n=94). Protein kinase Cd mRNA expression was significantly higher in oestrogen receptor ( ER)-positive compared with ER-negative tumours (P=0.007, Mann-Whitney U- test). Increasing concentrations of PKC delta mRNA were associated with reduced overall patient survival (P=0.004). Our results are consistent with a role for PKC delta in breast cancer progression.