The vasculature of the adrenal gland has been studied by microinjection techniques in a variety of species. While there is general agreement about the overall patterns, some uncertainty still exists over the structure of medullary arteries and the connections between the sinusoids of the cortex and medulla. We have taken a new approach to these problems by applying immunohistochemical techniques to the human adrenal gland, identifying overall vascular patterns by endothelial expression of CD34 and muscular channels by smooth muscle actin. We have also examined adrenal nodules, adenomas and carcinomas to see whether these can be differentiated on the basis of their vascular patterns. The general pattern in the normal gland was similar to that found in injection studies, but there appeared to be more connections between sinusoids of the zona fasciculata than previously reported. There was direct continuity between cortical and medullary sinusoids. Medullary arteries were demonstrated as thin-walled vessels. Immunopositivity for smooth muscle actin was present in sinusoids, apparently in endothelial cells, suggesting that they may express this protein and thus have a contractile function. Macronodules and adenomas could not be reliably distinguished, both showing a rich network of sinusoidal vessels. Carcinomas showed marked disorganization, with large-calibre vessels interspersed with irregular networks of vessels of very small calibre.