4 patients with hereditary hemorrhagic telangiectasia (Osler's disease) have been studied by clinical means and by fluorescence videomicroscopy. The capillaroscopic image was characterized by convolutes of giant capillaries (diameter up to 150 micron, normal value up to 15 micron) between capillaries of normal shape and size. After intravenous injection of Na-fluorescein the enlarged capillaries filled rapidly and in some loops flow direction changed at irregular intervals. These findings are in contrast to those observed in systemic sclerosis, where generalized microangiopathy with avascular fields, microbleedings, enlarged capillaries and increased transcapillary and interstitial diffusion of the fluorescent dye are typical. In borderline cases capillaroscopy contributes to differential diagnosis of the telangiectatic form of scleroderma and Osler's disease.