A case of uterine leiomyoblastoma is presented with ultrastructural and cytological findings. As in gastric leiomyoblastoma, fine filaments of dimensions consistent with myofilaments and marginal dense bodies are present within the tumor cells, providing evidence for the smooth muscle origin of this neoplasm. Numerous cytoplasmic vacuoles are seen in cellular samples obtained by directly scraping the lesion corresponding to the vacuoles present in glutaraledehyde-fixed material examined by electron microscopy. They do not appear to be artifacts of poor fixation and may be related to the nature of the tumor. Cytologically the cells do not resemble epithelial cells of either squamous or glandular origin and are more consistent with cells of mesenchymal origin.