To date, no criteria exist for anticipating the response of a port wine stain to argon laser therapy. In an effort to determine such predictive factors, the preceding study was undertaken. Sixty-two patients, ages 7 to 66 years, with port wine stains were biopsied, had a small test area treated, and were evaluated after 4 months. A desirable result, defined by marked lightening of the lesion without scarring, occurred in 73 percent of the patients. Factors favoring a desirable result included age greater than 37 years, purple color, fraction of dermis occupied by vessels greater than 5 percent, mean vessel area greater than 2500 micrometers2, and percent of vessels containing erythrocytes greater than 15 percent. Furthermore, the degree of color change strongly correlated with these indices. Factors indicating an undesirable result included age less than 17 years, pink color, vascular area less than 2 percent, mean vessel area less than 1500 micrometers2, and percent of vessels containing erythrocytes less than 3 percent, Thus, if appropriate clinical and histological criteria are met port wine stains can successfully treated with argon laser therapy.