Summary. The proportion of proliferating tumour cells in a tumour (‘growth fraction’) was estimated in 30 patients with primary ovarian carcinoma, using monoclonal antibody Ki-67 which reacts with a nuclear antigen present only in proliferating cells. A positive correlation between the ‘growth fraction’ and both the clinical stage and histological grade of the tumours was observed. A high ‘growth fraction’ was observed in advanced, metastatic and poorly differentiated tumours and a low ‘growth fraction’ with early stage, well-differentiated tumours. This information which has not been available previously to clinicians may be of practical value for determining the prognosis, and influencing treatment recommendation.