BACKGROUND Biochemical methods for estimating protein intake are based on the concept that nitrogen-containing products of protein in diet plus the products arising from endogenous protein are excreted as either urea or non-urea nitrogen (NUN). This formulation is based on the fact that the urea is the principal end product of amino acid degradation and, hence, the urea appearance rate (or net urea production) is parallel to protein intake. The urea nitrogen appearance (UNA) rate is measured as the amount of urea excreted in urine plus the net amount accumulated in body water. A more difficult problem is how to estimate NUN, the sum of fecal nitrogen, and all forms of non-urea urinary nitrogen. Maroni, Steinman, and Mitch (Kidney Int 27:58-65, 1985) proposed estimating nitrogen intake (IN MARONI) from UNA plus NUN excretion rate of 0.031 g nitrogen/kg body weight/day, as they found NUN correlated with body weight but not with dietary nitrogen. Kopple, Gao, and Qing (Kidney Int 27:486-494, 1997) proposed a different equation for estimating nitrogen intake (IN KOPPLE) = 1.20 UNA + 1.74, concluding that dietary nitrogen directly correlates with fecal nitrogen and that NUN is constant for all patients. Their report prompted us to review all nitrogen balance measurements we had conducted in order to address the following questions. Does dietary protein increase fecal nitrogen excretion? Does NUN vary with weight or is it constant? How do the two methods (IN MARONI and IN KOPPLE) compare in estimating dietary protein from UNA? METHODS We examined nitrogen balance and its components measured in 33 patients with chronic renal failure (CRF) who were eating diets varying from 4.1 to 10.1 g nitrogen/day. We evaluated relationships between dietary nitrogen [intake nitrogen (IN)], NUN, fecal nitrogen, body weight, and the predictability of the two methods. RESULTS Neither fecal nitrogen nor NUN were significantly correlated with IN (r = 0.04 and r = -0.07, respectively). NUN significantly correlated with body weight (P = 0.008). Measured IN averaged 5.75 +/- 0.41 g nitrogen/day; the estimated IN MARONI value was 5.61 +/- 0.27 g nitrogen/day; the estimated IN KOPPLE was 6.04 +/- 0.44 g nitrogen/day. The prediction errors associated with the IN KOPPLE equation were slightly but not statistically higher than that associated with IN MARONI. CONCLUSIONS Fecal nitrogen is not correlated with IN. NUN is not constant but varies with weight, and the traditional method of estimating IN in stable chronic renal insufficiency (CRI) patients from UNA and weight as proposed by Maroni, Steinman, and Mitch is valid.