To clarify the importance of the duration from the onset of a urinary abnormality until a biopsy is actually performed (UA-Bx time) in making a renal prognosis, we investigated 496 patients with IgA nephropathy (male/female: 222/274, mean age: 33.0 +/- 13.7 yrs, mean follow-up period: 10.8 +/- 4.3 yrs). All patients were found to have a urinary abnormality, including both hematuria and proteinuria, at clinical onset while demonstrating a normal renal function, and showing a serum creatinine level of < or = 1.2 mg/dl or a creatinine clearance level of > or = 80 ml/min. The UA-Bx time was divided into 3 groups: < 1 yrs (S-G), 1 < or = < 3 yrs (M-G), > or = 3 yrs (L-G). The severity of glomerular damage was divided into 5 groups based on the occupational rate of segmental sclerotic glomeruli. Based on a multivariate analysis of independent prognostic factors relating to renal death, the severity of glomerular damage was the most independent factor, while the UA-Bx time showed no risk for renal death. However, based on a multivariate analysis of the UA-Bx time regarding the timing of a renal biopsy, patients in L-G, which had the most glomerular damage, showed twice the hazard ratio as those in S-G or M-G and the difference was significant. These results thus indicate that because the glomerular damage is able to progress for 3 yrs or longer after the clinical onset of renal disease, a renal biopsy should therefore be performed within 3 yrs from the clinical onset in patients demonstrating both hematuria and proteinuria when such patients are also suspected of having IgA nephropathy.