The phosphatidylinositol 3-kinase (PI3K)/Akt signalling pathway plays a major role in the regulation of breast cancer growth and survival, but the clinical value of its components in human tumours is unclear. PI3K was analysed using Western blotting with monoclonal antibodies to the p85 subunit in tumour and adjacent mammary gland samples from 33 breast cancer patients. Activated Akt1 (pAkt1) expression was quantified in 46 sample pairs by a direct sandwich ELISA assay. Tumour PI3K expression was increased in 79% of the investigated sample pairs and was not associated with the main clinico-pathological features. Only 49% of breast cancers had increased pAkt1, but the frequency of its elevation was positively associated with tumour size and histological grade, and controversially related to estrogen and progesterone receptor status. Increased PI3K, but not pAkt1, expression appears to be a widespread feature of human breast cancer indicating the different roles of the two components of one signalling system.