This is the first ultrastructural study of a neoplasm arising in a parovarian Müllerian (paramesonephric) cyst. By light and electron microscopy this tumour bore a close resemblance to the ovarian serous cystadenoma of borderline malignancy. This resemblance to serous neoplasms of the ovary and to other tumours of Müllerian duct derivation was the strongest claim to its origin. Other features which indicated Müllerian origin included the absence of a well-defined basement membrane, the presence of distinct muscle coats and the relatively large size of the epithelial cells and their nuclei. We suggest that the same prognostic criteria used for ovarian serous tumours might be applicable to the present lesion and conclude that Müllerian adnexal tumours like their Wolffian counterparts can also be identified by their morphological characteristics.