Polymorphism of the gene that codes for angiotensin I-converting enzyme (ACE) is associated with increased severity of immunoglobulin A (IgA) nephropathy in adult patients. We evaluated the relationship between the polymorphism of ACE genotypes and the pathological and clinical findings in Japanese children with IgA nephropathy. Patients with moderate/diffuse mesangial proliferation, glomerular sclerosis and tubulointerstitial damage showed a significant increase of the D/D type compared to those who had mild/focal mesangial proliferation, without glomerular sclerosis or tubulointerstitial damage (p < 0.05). Proteinuria at the first renal biopsy was significantly higher in the former group compared with the latter group except glomerular sclerosis (p < 0.01). IgA nephropathy patients with tubulointerstitial damage also showed an increased serum creatinine level compared to patients without the damage (p < 0.03). We conclude that ACE gene polymorphism may be correlated with the prognosis of IgA nephropathy in Japanese children.