We have encountered three cases of a recently recognized form of granulosa cell tumor referred to by Scully as the juvenile type, and we have made ultrastructural observations on one case. This variant is encountered almost exclusively in the first two decades, and is characterized at the optical microscopic level by a macrofollicular or a diffuse, sometime disorderly pattern of growth, often with extensive luteinization and hyperchromatic nuclei. One of our patients developed precocious pseudopuberty. Our ultrastructural observations in this case supported the concept that this tumor is a granulosa cell tumor, and the tumor showed some similarities to previously reported granulosa cell tumors. A spectrum of cells was observed ranging from well differentiated granulosa cells to stromal cells with many intermediate forms present. The stromal cells varied from being fibroblast-like to theca-like. Both granulosa and stromal cells sometimes contained abundant lipid. Evidence of luteinization, i.e. abundant smooth endoplasmic reticulum and tubular mitochondrial cristae, was not noted in either the granulosa or stromal cells. Focal areas of smooth endoplasmic reticulum were found, however, in teh cytoplasm of cells intermediate between granulosa and stromal types. These cells may represent the source of steroid hormone secretion.