Despite similarities in their chemical structures, the new fluoroquinolones differ greatly in the extent to which renal clearance contributes to their elimination. On the opposite ends of this spectrum are ofloxacin, which is almost entirely eliminated by the kidney as unmodified drug, and pefloxacin, which undergoes extensive biotransformation. Dosage adjustments in patients with renal insufficiency therefore are recommended for ofloxacin, but, on the basis of currently available data, adjustments do not appear to be required for pefloxacin. Both renal clearance of unmodified drug and metabolic transformation contribute substantially to the elimination of norfloxacin and ciprofloxacin. Because of the great antimicrobial potency of the new fluoroquinolones, it should be possible to employ them in the treatment of urinary tract infections caused by highly susceptible bacteria even in patients with marked impairment of renal function.