**5. Conclusions**

The severity of mesangial C3 deposits in kidney biopsy rated > +1 has an effect on renal survival with normal GFR in children with IgAN. A reduced serum C3 level has not been a prognostic factor in this group of children but perhaps this finding should be confirmed in a larger group of children.

**Author Contributions:** Conceptualization, M.M.-W., A.S.-G. and K.C.-K.; methodology, M.M.-W.; validation, M.M.-W., A.S.-G. and A.T.; formal analysis, M.M.-W. and A.S.-G.; investigation, M.M.-W., A.S.-G. and K.C.-K.; resources, M.M.-W., A.S.-G., K.C.-K., A.T., J.M., M.M., D.D., A.F.-A., R.S., A.R.-S., A.W., M.S., B.B., P.S., A.P.-M., D.Z., M.P.-B., M.T., J.Z., M.D.-D., A.Z., K.G.-P. and R.G.; data curation, ˙ M.M.-W., A.S.-G., K.C.-K., A.T., J.M., M.M., D.D., A.F.-A., R.S., A.R.-S., A.W., M.S., B.B., P.S., A.P.-M., D.Z., M.P.-B., M.T., J.Z., M.D.-D., A.Z., K.G.-P. and R.G.; writing—M.M.-W. and A.S.-G.; writing— ˙ review and editing, M.M.-W., A.S.-G., K.C.-K. and M.P.-T.; supervision M.P.-T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Bioethics Committee at the Medical University of Warsaw (No. KB/147/2017).

**Informed Consent Statement:** Informed consent for study participation was obtained from the legal guardians of the study participants.

**Data Availability Statement:** The data analyzed in this study are available from the corresponding author on reasonable request.

**Conflicts of Interest:** The authors declare no conflict of interest.
