Adrenal pseudocysts are cystic lesions arising within the adrenal gland surrounded by a fibrous tissue wall devoid of a recognizable lining layer. This study comprised eight adrenal pseudocysts surgically excised at the Massachusetts General Hospital. The median age of the patients was 41 years. Seven of the eight individuals were women. There was no apparent etiologic relationship to prior trauma or pregnancy. Half of the patients described symptoms that resolved following pseudocyst removal. The remaining individuals were asymptomatic with adrenal pseudocysts discovered incidentally during the work up of other medical problems. Adrenal pseudocyst size ranged from 1.8 to 10 cm. Pseudocyst size did not correlate with the presence of symptoms. The histologic and immunohistochemical findings in this study suggest that at least some adrenal pseudocysts are of vascular origin. In two lesions, small foci of residual cells lining the inner pseudocyst wall were found that expressed factor VIII antigen (also Ulex Europaeus lectin in one case) but not vimentin, keratin, or epithelial membrane antigen. In both cases, abundant elastic tissue was present in the pseudocyst wall, and in one lesion, adrenal vein smooth muscle was present as well. In both cases, dilated sinusoids were found at the periphery of the pseudocysts and, in one lesion, the sinusoids appeared to coalesce to form the pseudocyst cavity. In the majority of the remaining six cases there was also some histologic evidence to suggest a vascular origin. In five and three cases, respectively, abundant elastic tissue and adrenal vein smooth muscle were found within the pseudocyst wall. In two lesions, both elastic tissue and smooth muscle were present. In addition, adrenal sinusoids were prominently dilated at the periphery of four pseudocysts and, in one case, the sinusoids appeared to coalesce to form the pseudocyst cavity.