Nitrogen balance has historically been estimated by using urinary urea nitrogen (UUN) multiplied by a factor of 1.25 to account for nonurea nitrogen present in the urine. Recently, the reliability of UUN as an estimate of nitrogen losses has been questioned and the use of total urinary nitrogen (TUN) has been proposed as a more accurate measure of urinary nitrogen losses. However, analysis of TUN losses is not readily available in many hospital laboratories. Because ammonia is the major fluctuating component of urinary nonurea nitrogen and equipment to measure urinary ammonia is available in most hospitals, this study was undertaken to determine whether urinary ammonia plus UUN provides a clinically useful approximation of TUN. Twenty-four-hour urine samples acidified with boric acid during collection from 20 patients (a total of 42 samples) receiving total parenteral nutrition were analyzed for UUN, ammonia, and TUN. The UUN values ranged from 4.9 to 42.4 g/24 h. The mean difference between TUN and UUN was 1.99 +/- 0.27 g/24 h. The mean difference between TUN and UUN plus ammonia was 0.78 +/- 0.27 g/24 h. Thus, UUN alone accounted for 90% and the combination of UUN plus urinary ammonia accounted for 96% of TUN. These data suggest that UUN plus ammonia does provide a greater level of reliability as an estimate of TUN than UUN alone.