Clinicopathological evaluation of flashlamp-pumped dye laser therapy for port wine stains was conducted on 474 subjects (135 male and 339 female) ranging in age from less than a year to 85 years (median = 17; lower quartile = 7 and upper quartile = 28). There was a significant variation in the rate of favourable response among lesion sites but the variation in relation to sex, age and radiation dose was not significant. Histological investigations of skin biopsy specimens from 23 subjects indicated that the mean depth of vessels is the main prognostic factor. The critical value (standard error) of the mean depth for a favourable response was estimated as 930(50) microns. Applications of this estimate to clinical prognosis are discussed.