In long-term intervention studies on renal function outcome an initial decline in the glomerular filtration rate (GFR) may occur after starting therapy. If this initial GFR decline is the result of a treatment-induced hemodynamic change reflecting a fall in intraglomerular pressure, it should be reversible after treatment withdrawal, even after long-term treatment. In fact, it could be beneficial for renal function in the long term. We therefore studied systemic and renal hemodynamics in 40 non-diabetic patients with impaired renal function before treatment, during four years treatment with either atenolol or enalapril, and after withdrawal of that treatment. The acute change in GFR 12 weeks after start of treatment varied widely from -11 to + 11 ml/min (mean +/- SD -1.0 +/- 4.1 ml/min, NS). After four years of treatment, withdrawal for 12 weeks resulted in a rise in GFR of +2.2 +/- 5.4 ml/min, P = 0.011, again with a wide range of +14 to -6 ml/min). The initial fall in GFR was related to the rise after withdrawal (r = 0.32, P < 0.05). Interestingly, the acute treatment induced change in GFR correlated with the long-term slope, such that a patient with a greater initial decline in GFR showed a more stable course during the follow up (r = -0.36, P < 0.05). The patients were arbitrarily divided in group A (N = 20), with the largest initial treatment-induced fall in GFR, and group B (N = 20), with the smallest initial fall in GFR. Group A had a significantly less steep slope than group B (-0.41 +/- 1.52 vs. -2.09 +/- 2.79 ml/min/year, P = 0.023) during the four year follow-up. In group A GFR increased again after withdrawal of treatment (+3.8 +/- 5.6 ml/min, P = 0.011) whereas it did not change in group B (+0.5 +/- 4.0 ml/min, NS). As a consequence, GFR post-treatment was not different compared to pre-treatment in group A (-2.5 +/- 7.2 ml/min, NS), whereas it was 5.9 +/- 12.1 ml/min lower in group B (P = 0.023). Patients treated with enalapril had a similar response as patients treated with atenolol. In conclusion, an initial fall in GFR after starting antihypertensive treatment in patients with a mild to moderate renal function impairment (GFR 30 to 90 ml/min) is reversible even after years of treatment, suggesting that this therapy-induced fall is of hemodynamic and not of structural origin. This initial GFR fall was associated with a subsequent stable renal function. These data lead to the hypothesis that the initial fall in GFR in response to antihypertensive therapy reflects renal protection.