Elevation of the ratio of glycine: taurine-conjugated bile acids (G/T ratio) is thought to contribute to fat malabsorption in cystic fibrosis (CF). The cause, extent, and reversibility of taurine deficiency in CF were assessed using balance studies in 6 subjects (ages 8-14 years) who were supplemented with taurine (0.24-2.4 mmol/kg/24 h) for 1 week. Taurine reduced the G/T ratio both in serum and duodenal juice in all children. The mean fecal taurine loss in CF subjects [10.8 mumol/kg/24 h +/- 9.9 (SD), range 0.9-27.9] was much greater than that in controls (less than 0.1 mumol/kg/24 h, n = 4) and approximated the dietary taurine intake (mean 14.6 +/- 4.4 mumol/kg/24 h, n = 12). Absorption of an oral taurine load appeared to be normal in CF. Excessive fecal taurine loss appears to predispose CF children to bile acid taurine deficiency, a deficiency that can be corrected by oral taurine supplements.