C3 glomerulopathy (C3G) is a very rare immunological (or more precisely: complement-mediated) inflammation of the glomeruli. Due to progressive renal dysfunction, many such patients have to go on dialysis or receive a kidney transplant after about ten years. C3G has to date been treated by lowering blood pressure and proteinuria and by non-specific immunosuppression. In order to compare different therapeutic approaches in the future, a “European Register for C3 glomerulopathy and immune-complex-mediated MPGN,” in which cases are systematically registered, was initiated in 2015.