Based on mathematical analysis, a method computing deviations from normal in body weight, water and sodium, and requiring only presenting body weight and hyperglycemic and euglycemic plasma glucose and sodium concentrations in anuria was developed. Data from 18 hyperglycemic episodes (12 anuric patients) treated with insulin alone were analyzed by this method and were compared to clinical estimates. When normal body water and extracellular volume were assumed to be 0.579 and 0.213 of dry weight, respectively, deviations of presenting weight from dry weight (in kg) were as follows (x = clinical estimate, y = new method estimate): x: -2.5 to +17.2 (+4.6 +/- 5.1, SD); y: -4.1 to +16.7 (+4.2 +/- 5.4, NS); y = -0.4 + 1.0008x, r = 0.96 (p less than 0.01). Similar comparisons were obtained for deviation of body sodium from normal and for euglycemic extracellular volume. Estimates by the two methods agreed when normal body water was assumed to vary between 0.525 and 0.625 of dry weight and when normal extracellular volume was assumed to vary between 0.31 and 0.40 of normal body water.