Twenty-three patients with facial port wine stains were studied to determine whether chilling lesional skin at the time of treatment could improve the outcome of argon laser therapy and whether this effect could be attributed to increased hemoglobin content of chilled sites, as hypothesized on clinical grounds. Each patient was biopsied in two representative and clinically identical sites, once at room temperature and once immediately after application of ice to the skin surface for 2 to 3 minutes. Two additional identical sites were treated with an argon laser in the same manner. Histologic sections of the port wine stain after application of ice tended to have a higher percentage of erythrocyte-filled vessels, but the effect of chilling on the dermal vasculature varied greatly among patients and was not statistically significant. In contrast, chilling of lesional skin prior to laser therapy resulted in a significantly better average outcome (p = 0.0002), with 57 percent of chilled sites superior to the paired room temperature control and none inferior. In nearly all instances of differential response, the site treated at room temperature manifested scarring, while the chilled site did not. Overall, after an average evaluation period of 4.8 months, 65 percent of the patients achieved a good or excellent result in the control site, and 87 percent achieved this result in the chilled site. These data establish the potential benefit of lesional modification prior to argon laser therapy and suggest that in the case of port wine stain chilling, this benefit is due to reduced heat injury of nonvascular elements in the skin.