The ultrastructural changes in the vessels of the fingertips of patients with Raynaud's phenomenon or disease, with or without scleroderma, are described for the first time. Fingertip specimens were taken by punch biopsy. The light microscopic changes, including segmental vasculitis, fibrinoid degeneration of capillaries, and involuntary regression of glomus bodies explain the reduced digital blood flow associated with Raynaud's disease and phenomenon. The moment-to-moment digital blood flow, recorded rheoplethysmographically, was low when the patient was in a comfortable environment and was increased in a hot environment. This result substantiates the therapeutic importance of reflex vasodilatation. These findings correlate well with the ultrastructural changes in capillaries, including multiple cytoplasmic folds in endothelial cells, abundant intracytoplasmic filaments, and unusual incorporation of collagen fibrils in the basement membrane. These changes must influence nutritional and thermoregulatory function of the digital blood vessels.