Polymorphs infiltrate glomeruli in mesangial IgA glomerulonephritis.During episodes of macroscopic hematuria, patients with IgA nephropathy commonly have polymorphonuclear neutrophils (PMNs) as well as fibrin and mononuclear cells in glomerular capillaries. We quantitated PMN and macrophage infiltration in glomeruli of 54 patients with IgA nephropathy in whom renal biopsies were obtained within 30 days of macroscopic hematuria. Control biopsies (N = 22) were from patients with IgA nephropathy and urinary erythrocyte counts below 50,000/ml. PMN monocyte/macrophages were quantitated using the monoclonal antibodies FMC10 and HAM56, respectively. In patients with heavy hematuria, 45.9 ± 3.4% (mean ± SE) of glomeruli were positive for PMNs (control 10.5 ± 2.8%, P = 0.001) with a mean PMN count/glomerulus of 1.10 ± 0.20 (control 0.13 ± 0.03, P = < 0.001). 65.6 ± 9.7% Of the glomeruli were positive for monocytes in the heavy hematuria group (control 40.0 ± 8.4, P < 0.05) with the mean monocyte count per glomerulus being 1.6 ± 0.2 (control 0.6 ± 0.1, P < 0.01). We conclude that, in the acute phase of mesangial IgA nephropathy, PMN and monocytes are present and presumably participate in glomerular injury.