A "rod-shaped tubulated body" (tubular body) was first described by Weibel and Palade in the vascular endothelial cells of various organs in both man and animals. This is now considered to be an organelle specific to the endothelial cell, but its function is still unknown. Both in experimental and human pathology this organelle has been observed more often in either seemingly young or abnormal endothelial cells of the blood vessels in tissue regeneration, inflammation, brain tumors among others. This report deals with ultrastructural study of two surgical cases of cerebellar neoplasm, in which the vascular endothelium was examined for a tubular body. The first case was a 12-year-old boy with cerebellar hemangioblastoma, and the second a 36-year-old female who had a history of renal cell carcinoma removed approximately 5 years previously. Histological diagnosis of the cerebellar tumor in the latter case was indetermined, because a part of the tumor consisted of clear cells suggestive of clear cell carcinoma and another part of well developed endothelial cells and vascular channels apparently indicative of hemangioblastoma. The findings of the ultrastructural study were rather compatible with that of renal cell carcinoma metastatic to the cerebellum although inconclusive. The tubular body observed in the endothelial cells of those tumor vessels consisted of a membrane-limited round, oval or elongated shaped intracytoplasmic body which contained tubules of 170 to 200 A outer diameter with approximately 50 to 60 A thickness. The tubules were arranged mostly in a parallel fashion along their long axis. In the first type of tubular body they were embedded in a relatively pale matrix, and in the second their arrangement appeared to be more compact. The third tubular body, so far undescribed in human endothelial cells except in our previous communication, showed an irregularly and markedly enlarged matrix, surrounded by a limiting membrane which was occasionally observed connected with either a coated vesicle or cytoplasmic membrane. Abundant tubules were intermingled without showing a particular arrangement. Morphological and functional significance of the third type tubular body is unknown, but it might represent a pathological change of a tubular body in cerebellar neoplasms. These findings might give us some clues in understanding a tumor angiogenesis.