Red cells of 3 patients with the painful bruising syndrome showed morphological abnormalities. In the fraction not sedimenting in Ficoll/Isopaque gradient centrifugation, some of the cells had club-shaped processes, mitochondria, nuclear remnants, and vacuoles. In freeze-etch preparations, 90% of the red cells showed membrane elevations at pH 7.4 and 25% at pH 6.4, while in freeze-etch preparations of controls these values were 55 and 0 respectively. In addition, rouleaux formation was markedly enhanced in the preparations of blood of patients with the painful bruising syndrome.