Estimations of total urinary nitrogen from measured urinary urea nitrogen are commonly used in calculating nitrogen balance. Recently published studies suggest the urinary urea nitrogen/total urinary nitrogen relationship is inconstant and total urinary nitrogen must be directly measured in burned patients. This study addresses the relationship of urinary urea nitrogen to total urinary nitrogen after thermal injury. Two hundred random 24-hour urine collections obtained from 45 thermally injured patients (mean burn size 59 +/- 28%, mean age 40.5 +/- 17.2 years) between 1 and 354 days postburn were analyzed for total urinary nitrogen and urinary urea nitrogen. Regression analysis relating total urinary nitrogen to estimated total urinary nitrogen (urinary urea nitrogen x 1.25) revealed a linear relationship (r = .936, p < .001). The mean urinary urea nitrogen/total urinary nitrogen ratio was 0.77 +/- 0.10 and was not significantly correlated with percent burn, age, or postburn day. Mean nitrogen balance calculated from measured urinary urea nitrogen in these patients was -5.7 g, and that calculated from measured total urinary nitrogen was -6.3 g. This difference, although statistically significant, is of little consequence for clinical use. Contrary to recent reports, we found the urinary urea nitrogen to be sufficiently predictive of total urinary nitrogen for practical application, and do not consider routine total urinary nitrogen measurements necessary for the nutritional care of thermally injured patients.