Studies have shown port-wine stains (PWSs) probably represent an aneurysmal dilation and ectasia of the cutaneous vascular plexus. The abnormal vessels are largely included within a 0.6-mm subepidermal zone, which is within the argon laser destruction range. Twenty-eight patients with PWSs underwent biopsies prior to argon laser treatment and repeated biopsies 4 1/2 months later. Those lesions (23) that contained large ectatic blood-filled vessels responded well to argon laser therapy in contrast to those (five) in which the vessels were relatively small and erythrocyte free. A similar histologic pattern was seen in the repeated biopsy specimens of both groups: the mean vessel area, the fraction of dermis occupied by vessels, vascular erythrocyte content, and vessel wall thickness sharply decreased, while the number of vessels tripled. Since lesion color strongly correlates with and probably is largely determined by erythrocyte content, the reason for the PWS lightening, despite increased vessel number, is the relative lack of erythrocytes in the newly formed vessels.