Previous ultrastructural studies on follicular thyroid carcinomas present conflicting results, in particular with regard to the surface features of the neoplastic cells. The present study of 9 cases with varying degrees of differentiation is the first comprehensive investigation by concurrent transmission and scanning electron microscopy. In the well-differentiated tumours we found a slightly increased number of microvilli and a decreased number of cilia per cell as compared to the normal thyroid follicles, while moderately differentiated tumours had a smaller number of microvilli and no cilia. The cells in anaplastic foci had unimpressive surfaces with only scattered clusters of short microvilli. Neither scanning nor transmission electron microscopy seem to be of major importance in the diagnostic evaluation of the individual case of follicular thyroid carcinoma or in the assessment of prognosis. Scanning electron microscopy presents a wealth of information on the three dimensional surface arrangement of the neoplastic cells. However, in a tumour where surface alterations may reflect both changes in endocrine function and malignant potential, the interpretation of isolated morphological findings is extremely difficult.