The interpretation of traditional serum urea and creatinine concentrations as indices of the severity of uremia requires major modifications in hemodialyzed patients. Although high urea concentrations usually signify worsening uremia and inadequate dialysis, low concentrations do not guarantee a good outcome. Urea production as modified by diet and other factors must also be included in a complete description of dialysis quantity and adequacy. The expression 'Kt/V' is a measure of hemodialysis that includes both urea removal and urea generation and is easy to measure from predialysis and postdialysis serum urea concentrations. Kt/V can be most precisely measured with the aid of mathematical models of urea kinetics during and between hemodialyses. Although a reliable measure of the dialysis dose received by most patients, the single-compartment model overestimates serum urea concentrations during hemodialysis and fails to predict the rebound immediately following dialysis. The classic two-compartment model that includes a factor for resistance to diffusion between the compartments, more accurately predicts the BUN profile but fails to account for blood flow-related disequilibrium including cardiopulmonary recirculation. Since solute disequilibrium reduces the effectiveness of hemodialysis, models that incorporate equilibrated urea concentrations both before and after hemodialysis are potentially more accurate tools for quantifying dialysis. Dialysate methods have the potential to accurately measure both solute removal which is the ultimate goal of dialysis, and patient clearance which is considered a better measure of the dialysis effect than dialyzer clearance. Application of these newer techniques requires major changes in sampling methods and changes in analytical equipment that will delay implementation. Meanwhile, analysis of blood-side urea concentrations using the single-compartment, variable volume model provides a reasonable estimate of Kt/V but must be interpreted with due consideration of its well-recognized pitfalls.