Background: Serum PIIINP is a collagen III precursor that has been linked to many inflammatory diseases ending in fibrosis such as burn, scleroderma and liver cirrhosis. The study aimed at assessment of serum PIIINP in acne scar patients in adults < 18 years. Methods: The study was conducted on 30 patients attending at Dermatology Outpatient Clinic of Minia University Hospital. in the period from January 2022 till November 2022. Patients were divided into 2 groups: 15 patients with acne scars and 15 individual as a control group, Serum PIIINP was assessed in each patient using Human procollagen type 3 ELISA Kit from BT LAB (Bioassay Technology Laboratory), China, and by using of Huma reader 3700, Germany. Results: Acne scar patients had significantly higher levels of serum PIIINP, compared to that of control group. Conclusion: Serum PIIINP is predictor for ongoing scarring processes in acne scar patients and can be a useful serum biomarker in assessment of acne scar patients