Objectives Serum propeptides of type III and type VI collagen (PRO-C3 and PRO-C6) are elevated in advanced nonalcoholic fatty liver disease, but their value in patients with severe obesity and their evolution after bariatric surgery (BS) is unknown. It is unclear if these markers of fibrogenesis are affected by adipose tissue fibrosis (ATF). We studied the association of PRO-C3 and PRO-C6 with liver fibrosis before BS, examined their evolution after BS and how much patients’ ATF contribute to their levels. Methods Serum PRO-C3 and PRO-C6 were measured in 158 BS patients and compared with liver, subcutaneous and omental adipose tissue histology obtained during surgery. PRO-C3 and PRO-C6 levels of 63 patients were determined in follow-up at 3 and 12 months post BS. Results Patients in the highest quartile of PRO-C3 had a higher risk of advanced liver fibrosis (stage F3-4; odds ratio 5.8; 95% CI [1.5-29.9]; p=0.017) compared to the lowest quartile (adjustment for age, gender and BMI). PRO-C3 was positively correlated with markers of insulin resistance and liver enzymes. After BS, PRO-C3 levels decreased in patients with high baseline liver fibrosis. This decrease correlated with improvement of metabolic and liver parameters. PRO-C6 was not related to stage of liver fibrosis. ATF did not correlate with PRO-C3 or PRO-C6 levels at baseline or after BS. Conclusions PRO-C3 was associated with advanced liver fibrosis in patients with severe obesity, and decreased after BS, without being affected by ATF. These data suggest that BS prominently eliminates drivers of hepatic fibrogenesis in NAFLD.