The long term use of cyclosporin in renal transplant recipients may be complicated by chronic nephrotoxicity, evidenced by renal functional deterioration and suggestive histological lesions. In 11 renal transplant recipients treated with cyclosporin since 28 +/- 5.8 months, we reduced (n = 6) or stopped (n = 5) this drug after chronic nephrotoxicity was diagnosed. Five months later, we conducted hemodynamic studies and observed significant increases in renal plasma flow (I131 hippuran clearance from 239.5 +/- 106 to 327 +/- 50 ml/min/1.73 m2) and glomerular filtration rate (DTPA-TC clearance from 43 +/- 15 to 67 +/- 10 ml/min/1.73 m2) and a decrease in renal vascular resistances. We suggest that cyclosporin-associated chronic nephrotoxicity is accompanied by some degree of reversible vasoconstriction, or that histological lesions, particularly cyclosporin arteriolopathy, can disappear after cyclosporin withdrawal.