On pages 2428 to 2433 of this issue, Miyazaki et al. provide evidence that complement component C3 is synthesized within the glomeruli of patients with mesangial glomerulonephritis. Although it has been known for several years that cells isolated from the human kidney can synthesize a range of complement components, and that immune complex disease can increase the expression of these genes, it is only now that the link between intraglomerular synthesis of complement and human glomerulonephritis can be made. In view of the long interest in the role of complement in the pathogenesis of glomerulonephritis, this begs the question to what extent local complement synthesis may be involved in the generation of tissue injury. Given also the double-sided nature of complement-proinflammatory and protective-It remains to be seen if the roles of local and systemic complement differ in these respects. To weigh these questions, we examine the current evidence that connects local complement synthesis with renal injury, and ask what studies are needed to provide more definitive answers.