A port-wine stain (PWS) is a congenital capillary abnormality with an incidence of 0.3-0.5%. Although several other types of lasers have been used to treat PWSs, few studies have focused on the factors that affect the outcome of 577-nm yellow lasers. We aimed to assess the efficacy and prognostic factors affecting the PWS treatment by 577-nm yellow laser. This study was carried out on 42 patients with PWS. Each patient received 6-10 treatment sessions with a 577-nm yellow laser at 4-week intervals. Treatment efficacy was considered adequate when improvement of ≥ 80% of the lesion occurred. After treatment, marked improvement occurred in 7 (16.7%) patients, moderate improvement in 17 (40.5%) patients, mild improvement in 10 (23.8%) and 8 (19%) patients showed poor improvement. There was a significant association between improvement and gender of the patients and site of lesion (p = 0.028, p = 0.001, respectively). However, linear regression analysis showed that the site of the lesion can significantly predict the improvement (p <
0.001), while other baseline characteristics were not associated and cannot act as predictors for improvement. Yellow laser is a successful therapy choice for PWS, with a statistically significant improvement and minimal adverse effects. No significant association was found between improvement following laser therapy and baseline parameters, except for the location of PWS.