Effective use of diuretics in critically ill newborns requires an understanding of the ontogeny of renal function, a knowledge of the pathophysiology of the patient's disease, an expectation of the pharmacodynamics of the several classes of diuretics, and an appreciation of the kinetics and toxicities of the selected agents. Unfortunately, the number and scope of clinical pharmacologic studies of diuretics in critically ill newborns are inadequate for optimal therapy with most of these drugs.