In this study we examined the rate of decrease in size of facial port wine stains (PWS) as a function of number of treatments, lesion size, lesion location and patients' age. This study was performed at the University of Colorado Hospital Outpatient Dermatology Center, Denver, U.S.A. A consecutive sample of 91 patients 18 years of age or younger with facial PWS in which the entire lesion was treated at each visit were studied. Included were all patients who had a minimum of five treatments or complete clearance of their lesion in fewer than five treatments. Patients were evaluated following one, five and 10 treatments with the pulsed (450 s) dye (585 nm) laser. Improvement was defined as the percentage decrease in the size of the PWS. For all patients, the first five treatments resulted in a mean decrease in size of 55% while the second five treatments (38 patients) only improved the mean decrease in size by 18%. Grouped by location, the mean decreases in size from the first five and the second five treatments were as follows: central forehead = 100%, 0%; peripheral face = 58%, 28%; central face = 48%, 14%; and mixed (combination of peripheral and central face) = 21%, 9%. All central forehead PWS completely cleared within five treatments while none of the mixed PWS did so even with an average of 14 treatments. Grouped by size, mean decrease in size was highest for small lesions; < 20 cm2 = 67%, 21%; 20 to < 40 cm2 = 45%, 8%; and > 40 cm2 = 23%, 29%. Grouped by age, mean decrease in size was highest for young children: < 1-year-old = 63%, 33%; 1 to < 6 years = 48%, 15%; and older than 6 years = 54%, 10%. For all patients studied, maximal improvement was obtained in the first five treatments. Major determinants of treatment response in order of decreasing importance are PWS location, size and patients' age. The most successful responses are seen in young patients (less than 1 year old) with small PWS (under 20 cm2) that are located over bony areas of the face such as the central forehead. These three determinants may be useful tools to guide patient expectations and to predict the rate of improvement of PWS to pulsed dye laser treatment.