Review of histopathological material in nine autopsies and 35 skin biopsy specimens of Kaposi's sarcoma in male homosexuals suggested that aberrant lymphaticovenous connections occur in the earliest stage of the Kaposi lesion. Venular glomeruloid structures in the dermis and their analogous radial venolymphatic channels in medium-sized and larger veins signified coupling of the lymphatic and venous systems, a characteristic previously noted in angiographic studies and considered to be unique in Kaposi's sarcoma. Lymphatic channels penetrated veins selectively rather than arteries, particularly in deep fat, liver, gastrointestinal submucosa and the hilum of lymph nodes. The initiation of the Kaposi lesion thus may be an abnormal recapitulation of the coupling of venous and lymphatic systems which occurs during embryonic growth. A chronological staging scheme is used which proposes lymphaticovenous union as the initial morphological differentiating event. The precise origin of the characteristic spindle cells in the developing lesion remains unclear, although convergent differentiation of lymphatic and blood vascular endothelium may be considered. Alteration of the microcirculation, particularly that distal to the capillary bed, may explain several of the histopathological and haemodynamic features of Kaposi's sarcoma, including lesional thrombosis and infarction, tissue haemorrhage, vascular dilatation, cavernous pseudoangiomas and acute right-sided heart failure.