Measurement of the fractional excretion of sodium ions (FENa) and the urinary specific gravity (SG) aids considerably in the clinical assessment of fluid and Na+ balance. However, these determinations may not provide meaningful information if the patient has been treated with a diuretic. In particular, osmotic agents can affect circulating volume by causing natriuresis with a relatively high urinary SG, even in the face of volume and Na+ depletion. Two methods used frequently to identify osmotic diuretics were examined for accuracy. A new equation using the last two digits of SG was derived to identify osmotic agents without the need for an independent measurement of osmolality. The new equation was clearly superior to the more traditional formulas in that it identified accurately the presence of sodium diatrizoate, an ionized molecule, in each of 12 samples, whereas this contrast material was detected in only one sample by determining the difference between measured osmolality and the sum of the usual urinary constituents. A simplified version of the new formula was statistically as accurate as the more complex equation.