Two typical cases of sclerosing stromal tumours of the ovary are studied ultrastructurally, and the findings are correlated with the clinical data. One of the cases, with predominantly fibroblastic morphology, was asymptomatic. The second case, rich in cells with steroid-secreting features, had polymenorrhoea disappearing after removal of the tumour. The Authors suggest that clinical symptoms of sclerosing stromal tumour of the ovary depend from the degree of differentiation reached by the tumour cells.