In this study the efficacy and safety of long-term losartan administration on renal haemodynamics were evaluated in mild to moderate hypertension. After a run-in period with placebo, 18 hypertensives without renal or cardiovascular disease were allocated to losartan (50 mg/die for one year) treatment. Renal haemodynamic measurements included renal plasma flow (ERPF) and glomerular filtration rate (GFR) by standardized radioisotope study. Effective renal blood flow (ERBF), filtration fraction (FF), and renal vascular resistance (RVR) were also calculated. Blood pressure was evaluated monthly, whereas renal haemodynamics and function were detected at baseline and after 6 and 12 months of losartan administration. Losartan induced a significant (p < 0.001) decrease in SBP, DBP, and MBP versus baseline values both at 6 months and at 12 months. In addition a significant decrease in RVR (p < 0.001) and in FF (p < 0.05) was also seen. In addition RVR values at 1 year of treatment were higher than their values at 6 months, but this difference was not significant. Our data indicated that long-term control in blood pressure induced by losartan administration was associated with a maintained renal function after 6 months of treatment, but these favourable effects were attenuated after 1 year of treatment.