Histologic, pharmacologic, and exchange transplant studies were performed on a patient with nevus anemicus. Histologic examination disclosed no abnormalities. The only pharmacologic stimulus that produced erythema within the nevus was the alpha-blocking agent, phentolamine mesylate. Results of the transplant studies demonstrated donor dominance. This suggests that the defect in the nevus anemicus is attributable to increased sensitivity of the blood vessels to catecholamines rather than to increased alpha-adrenergic stimulation.