Cyclin E is a G1 cyclin which has been proposed to be one of the key regulators of the important G1/S transition, and could consequently be a potential deregulated molecule in tumours. Recently, it has been observed that cyclin E is overexpressed in a variety of malignancies including breast cancer and that several isoforms of the protein exists. In this study we have characterised the cyclin E expression in 114 tumour specimens from patients with primary breast cancer using Western blotting. Various expression of cyclin E was observed among tumours and a group of 27 patients out of 100 patients with stage I-III disease, identified as having tumours with high cyclin E levels, had a significantly increased risk of death and relapse from breast cancer (P = 0.0002 and P = 0.015 respectively). Even in the subgroup of axillary node-negative patients the cyclin E level was of prognostic importance. There was also a strong association between cyclin E expression and oestrogen receptor status (P < 0.00001), and tumours with high cyclin E expression were in general oestrogen receptor negative, suggesting a potential role for cyclin E in mechanisms responsible for oestrogen-independent tumour growth.