A capillary hemangioblastoma from the vermis of a 21-year-old man with von Hippel-Lindau's disease was successfully maintained in organ culture systems on Millipore filter platforms and gelatin sponge foam matrices up to 48 days. Three cell types--endothelial cells, pericytes, and stromal cells--identified by their fine structural features and their architectural relationships to vascular lumens and to the extracellular space were recognized in the original tumor. By light microscopy the explants showed, in the late stages, increased lipid droplets in the stromal cells and perivascular hyaline thickening. By electron microscopy, endothelial cells, pericytes, and stromal cells remained distinguishable as cell types. However, the stromal cells demonstrated, in addition to an increase of their normal complement of microfilaments and lipid droplets, features that were more characteristic of the two other cell types, i.e. micropinocytotic vesicles and the formation of zonula occludens junctions and hemidesmosomes. Basement membranes also became more apparent around the stromal cells. With increasing time in vitro, there was a striking increase in mature collagen fibers in the extracellular space. The roles of the different cell types in capillary hemangioblastoma and the histogenesis of the stromal cells are discussed in light of these observations. It is concluded that the capillary hemangioblastoma consists of multiple cell lines--endothelial cells, pericytes, stromal cells and, occasionally, hematopoietic cells--all of which are neoplastic and replicate in parallel with one another. Stromal cells may be regarded as an aberrant monopotential cell type which shares with the endothelial cell and pericyte a common mesenchymal, presumably angiogenic, ancestry, and may, on occasion, display morphological features, such as increased basement membrane formation and the formation of zonula occludens junctions, which recall its angiogenic lineage. However, interconvertibility between endothelial cells and stromal cells does not appear to occur in vivo or in vitro.