We evaluated the renal hemodynamics and the urine protein excretion rates of 73 children with Henoch-Schonlein nephritis (HSN). In 40 children we also performed a renal biopsy. The glomerular filtration rate (GFR) and effective renal plasma flow were determined by the clearances of inulin and para-aminohippurate during water diuresis. Urine albumin and IgG excretion were assessed in short-term timed samples. The mean GFR at the first examination was reduced in the HSN patients and most reduced in those with nephrotic proteinuria. There was an inverse correlation between the GFR at the first examination and the amount of albuminuria and urinary IgG excretion. Among the 40 patients with some degree of proteinuria who underwent a renal biopsy, 9 of 13 patients with mild to moderate proteinuria had severe morphological changes. GFR correlated inversely and fractional albumin and IgG excretion directly with the severity of the pathological findings on the biopsy, and with segmental and global sclerosis, the grade of mesangial proliferation, and interstitial inflammation. In conclusion, GFR is moderately reduced early in HSN and more reduced in patients with more proteinuria and in those with more advanced morphological changes. Moreover, even mild to moderate proteinuria may indicate severe morphological changes, which increase the indications for early renal biopsy in these patients.