Phospatidylinositol 3-kinase (PI 3-kinase) expression was analysed by Western blotting with monoclonal antibodies to the p85 subunit in a series of tumour and adjacent mammary gland samples collected at surgery from 33 breast cancer patients. Seventy-nine percent of the investigated pairs of the samples were characterised by an increased level of PI 3-kinase in the tumour in comparison with the adjacent mammary gland. PI 3-kinase activation was not associated with tumour steroid receptor status, histologic grade and other clinico-morphological characteristics. Furthermore, immunoblotting of epidermal growth factor receptor (EGFR) in the tumours with increased PI 3-kinase and corresponding adjacent tissues revealed no association between EGFR and PI 3-kinase activation. Thus, increased PI 3-kinase expression appears to be a widespread feature of breast cancer not associated with the main biological markers of its prognosis and hormone sensitivity.