Diffuse mesangial hypercellularity (DMH) is a rare primary mesangial proliferative glomerulonephritis associated with idiopathic nephrotic syndrome (INS). We conducted this study on 15 patients, including 5 patients with repeated specimens, with a follow-up of 0.9-17.5 years and evaluated the clinical course and pathological findings. Seven patients were male. Ten patients were under 14 years of age. All specimens had INS and were diagnosed morphologically with primary diffuse mesangial proliferative glomerulonephritis at initial biopsy; 4 were diagnosed with focal segmental glomerulosclerosis (FSGS) within 3 years by the second biopsy. The remaining 11 patients included 8 initial responders and 3 initial nonresponders to 8 weeks' steroid therapy and had the histologic variant of the minimal-change nephrotic syndrome (MCNS). Ten of the 11 patients had normal renal function during the investigation period. One patient with the MCNS variant who was refractory to steroid therapy developed end-stage renal disease (ESRD) within 6 years. Four patient with the histologic variant of FSGS included 1 initial responder, 2 late responders, and 1 steroid-refractory case. One patient with the FSGS variant developed ESRD within 4 years. The follow-up biopsies documented that the severity of mesangial hypercellularity was associated with the severity of proteinuria or hematuria. We conclude that DMH may be divided into heterogeneous disease entities, whereas morphologic changes in initial biopsies were similar. Each variant as well as the degree of DMH should be recognized routinely by follow-up biopsy, because they are prognostic indicators.