One benign Brenner tumour and one Brenner tumour of borderline malignancy were investigated by electron microscopy. The cells of the benign Brenner tumour nests and the cells in the borderline tumour were similar in ultrastructure. The intercellular spaces were large and reinforced by a moderate number of desmosomes. The nuclei were round or oval. The nuclear membrane was irregular in shape with deep infoldings corresponding to the characteristic nuclear groovings seen by light microscopy. Only few secreting cells could be found in the benign of Brenner tumour. The cystic cavities of the borderline Brenner tumour were lined by nonciliated secreting and ciliated nonsecreting cells. The secretory granules were PAS-positive and diastaseresistant. The granules stained homogeneously and strongly with the PASM-method at the electron microscopical level indicating the presence of 1.2-hydroxyl groups. The Brenner tumours have many similarities to the transitional epithelium and to the Muellerian-derived tubular structures. The findings support the theory that Brenner tumours are of coelomic origin and develop by direct metaplasia from the ovarian surface epithelium.