Abstract: 【Abstract】 Objective To evaluate the efficacy and safety of the 755-nm picosecond laser in the treatment of café-au-lait macules in children, and to analyze factors influencing the efficacy. Methods This retrospective cohort study included pediatric patients with café-au-lait macules who received 755-nm picosecond laser treatment at the Capital Center for Children's Health, Capital Medical University between February 2023 and January 2025. Appropriate energy density （1.59 - 6.37 J/cm2） was selected based on the patients′ ages, skin color, as well as lesion areas and color. The laser treatment was carried out at a frequency of 10 Hz, with spot diameters of 2 - 4 mm, and at 2- to 4-month intervals. Patient demographics, treatment parameters, and adverse reactions were recorded. Three dermatologists independently assessed efficacy according to pre- and post-treatment photographs, and the efficacy was classified into 4 grades: complete remission, marked improvement, moderate improvement, and no response. The Mann?Whitney U test and the Kruskal?Wallis H test were used to compare efficacy among patients with different characteristics （gender, age at initial treatment, lesion shape, lesion border, pigment distribution, lesion color, lesion area, and number of treatments）. A multivariate ordinal logistic regression model was used to identify independent predictors of treatment response. Results A total of 73 children with café-au-lait macules were included, comprising 29 males （39.73%） and 44 females （60.27%）. The age at initial treatment was 4.44 ± 3.44 years, ranging from 1 month to 12 years and 2 months; the number of treatments （M ［Q1, Q3］） was 3 （2, 4）, ranging from 2 to 6. At 2 - 4 months after the final treatment, 10 patients （13.70%） achieved complete remission, 31 （42.47%） achieved marked improvement, 20 （27.40%） achieved moderate improvement, and 12 （16.44%） showed no response, resulting in an overall response rate （complete remission + marked improvement + moderate improvement） of 83.56% （61/73）. Subgroup analysis revealed significantly better outcomes in lesions with irregular shapes, well-defined borders, and homogeneous pigment distribution than in those with regular shapes, poorly-defined borders, and heterogeneous pigment distribution （all P < 0.05）, while the efficacy was poorer in dark brown lesions than in brown and light brown lesions （P < 0.05）. Multivariate ordinal logistic regression revealed that the efficacy in lesions with irregular shapes was superior to that in lesions with regular shapes （adjusted OR = 13.839, 95% CI: 4.457 - 42.970, P < 0.001）, and the efficacy in lesions with poorly-defined borders was inferior to that in lesions with well-defined borders （adjusted OR = 0.199, 95% CI: 0.042 - 0.947, P = 0.043）. Adverse reactions included hyperpigmentation （4 cases, 5.48%）, hypopigmentation （1 case, 1.37%）, and mixed hyperpigmentation and hypopigmentation in different treatment areas （1 case, 1.37%）. Conclusions The 755-nm picosecond laser could be a safe and effective therapy for café-au-lait macules in children, with a low incidence rate of adverse reactions. Irregular shapes and well-defined borders may predict better treatment outcomes of lesions.

Key words: Cafe-au-lait spots, Child, 755-nm Picosecond laser, Treatment outcome, Influencing factors