Background <p>C3 glomerulopathy (C3G) is a rare kidney disease due to a dysregulation of the alternative complement pathway, orphan of specific treatment. Pegcetacoplan is an inhibitor of the third complement component C3, currently on a phase III registration protocol in C3G. Here we describe our experience with the off-label use of pegcetacoplan in pediatric patients with C3G.</p> Methods <p>This retrospective, observational study evaluated the efficacy and safety of pegcetacoplan in five pediatric patients, not eligible in the registration protocol, over a 12-week treatment period. The drug was given subcutaneously, twice a week for the first month, then weekly. The change in urinary protein-to-urinary creatinine ratio (mean of three samples) was the primary endpoint. We also evaluated the changes in serum C3, albumin, sC5-b9, creatinine, and urinary erythrocytes (number/µL).</p> Results <p>At baseline, median proteinuria/creatininuria ratio (mean of three samples) was 4.97&#xa0;mg/mg (3.53–7.69), and after 12&#xa0;weeks of treatment with pegcetacoplan, it decreased to less than 30% of baseline (<i>p</i> = 0.043) as did erythrocyturia (<i>p</i> = 0.043). C3 levels increased more than 600% of baseline (<i>p</i> = 0.043), whereas the levels of sC5-b9 decreased to normal range (<i>p</i> = 0.043). Three of four patients with impaired kidney function showed an improvement in eGFR. No adverse event was recorded.</p> Conclusions <p>In C3G patients, pegcetacoplan therapy improves clinical and laboratory features during a 12-week treatment. The present study, although small and with a limited follow-up, supports the use of complement-targeted therapy in C3G. Further studies with a larger number of patients and longer follow-up are needed.</p> Graphical abstract <p></p>