Photoelectric quantitative evaluation of argon laser treatment of port wine stains. Sixty patients with port wine stains were treated with the argon laser and graded visually into four subjective categories for changes in skin colour. For objective quantitative evaluation a photoelectric colorimeter was used. Reproducible definition of the lightness of the lesions into numbers was obtained. Differentials in lightness between normal contralateral skin and the port wine stain before and after treatment were quantified for each individual. The objective evaluation revealed a statistically significant difference between the four categories (P less than 0.05). The best clinical therapeutic effect was found to correlate with only 80-90% blanching. The average lightening obtained was 46.5%. Local heating of port wine stains in 12 patients showed no measurable darkening in the colour of the lesions, whereas the normal contralateral skin reacted with marked reddening. Determination of changes in lightness by photoelectric colorimetry provides an objective, quantitative means to evaluate the effects of laser treatment of port wine stains. 