Male spontaneously hypertensive fawn-hooded (FH) rats can be divided into two classes on the basis of their proteinuria. We investigated the relationship between early proteinuria and the impairment of renal function later in life. Urinary protein excretion (UpV), systolic blood pressure (SBP) and parameters of renal function were sequentially determined in male FH rats. A significant difference in UpV was already present from the age of 10 weeks. Class I rats excreted less protein than class II animals. Initially, the glomerular filtration rate (GFR) of class II animals was somewhat higher than that of class I rats, but from week 50 onwards a decrease in GFR of class II rats was noted and from then on rats died due to uraemia. The GFR of class I animals fell from week 75. The fall in GFR was preceded by a concomitant increase in UpV and SBP. The increase occurred earlier and the rate of increase was higher in class II rats. We conclude that a number of hypertensive FH rats die prematurely from end-stage renal failure. The presence of an increased UpV at an early age is an early marker for the development of renal failure.