In an attempt to understand the histogenesis and evolution of port wine stains (PWS), 100 patients with PWS were biopsied; the gross features of each lesion and the patient's associated clinical characteristics were recorded. A detailed analysis of each biopsy including both vessel and nonvessel parameters was made with the assistance of a computer. The central abnormalities characterizing port wine stains are an increase in vessel number (vascular profiles) and ectasia. Vessel number is highest in the immediate subepidermal area and then rapidly diminishes; mean vessel depth is .46 +/- .17 mm. In contrast mean vessel area shows less variation throughout the dermis, ectatic vessels being present when vessel number is very low. The product of both factors determines the percent of dermis occupied by vessels, but the mean vessel area is the major determinant. While age correlates poorly with vessel number, it correlates well with both progressive vessel ectasia and color shifts (pink to purple). Each of multiple vessel parameters analyzed (vessel number, mean vessel area, wall thickness, angulation, and luminal erythrocyte content) exhibited strong layer to layer correlation within the first .8 mm of tissue beneath the epidermis, indicating homogeneity of vessel characteristics within the lesion. The size of the lesion and facial quadrant distribution do not change with age nor are they related to any histological parameters. However the PWS lesion is found most often on the right side and lower quadrants, with a distinctive pattern being present in patients with glaucoma and mental retardation.