Our present histological knowledge of the port-wine stain is limited because of the small size of biopsies taken in the past. During the last two decades, Clodius has performed subtotal excision of port-wine stains, and has covered the defects with full thickness skin grafts. The availability of large surgical specimens of dermis affected by port-wine stain prompted the present study. Specimens from 50 locations were obtained in 28 patients. The pathomechanism of the port-wine stain was found to be telangiectasia of normal capillaries, sometimes congenital abnormal increase of the number of cutaneous vessels, or both. The theory of gradual age-dependent transformation of port-wine stain is true in regard to the colour of the affected skin, but is not valid in regard to the parallel increase of the vessel lumina and RBC filling grade. One-third of the patients below the age of 30 had the wide type lesion. The extreme histological differences in the size of the abnormal vessels, and the percentage of vascular area filled with RBC's were repeatedly noted in the different regions of the face in the same patient. Furthermore, even within the same area, the histology of port-wine stain varies greatly. The value of the small biopsies presently used for the assessment of laser treatment for these lesions is questioned.