The revascularisation process of transplanted human normal, hyperplastic and adenomatous parathyroid tissue was analysed at 2 and 4 days and 1, 2, 4, 7 and 12 weeks after transplantation to athymic mice. The transplants were examined by light and electron microscopy, immunohistochemistry and autoradiography. Vessels were detected by monoclonal antibodies specific for mouse and human endothelial cells. Immunohistochemistry demonstrated ingrowth of vessels from the host into the transplant and at one week numerous capillary sprouts were observed in the peripheral parts of the transplants. During the first week, peak levels of proliferation (labelling index) were observed in endothelial cells and capsular fibroblasts, and the proliferative capacity of endothelial cells was most pronounced in adenoma transplants. Fenestrated capillaries were observed in hyperplastic and adenomatous transplants, but not in transplants of normal tissue. In conclusion, revascularisation of transplanted human parathyroid tissue is enabled by ingrowth of vessels from the host into the transplant. The proliferative capacity of endothelial cells is higher and the process of maturation is faster in hyperplastic and adenomatous tissue compared to normal tissue.