The steatocrit method has recently been introduced as a simple screening test for steatorrhea. As it seemed likely that separation of fecal homogenate by centrifugation into a lipid phase, a watery phase, and a solid phase would be pH-dependent, we evaluated the effect of fecal acidification on steatocrit results. We also compared classical and acid steatocrit results in healthy children and in patients with cystic fibrosis and studied the relationship between two steatocrit methods and fecal fat content as measured by a reference chemical method. Steatocrit results increased with the degree of fecal acidification, and maximal results were obtained at the lowest fecal pH values. Means and SEM for classical and acid steatocrit values were 1.1 +/- 0.4% (classical) versus 3.8 +/- 1% (acid) in controls (n = 6) and 5.4 +/- 1.9% (classical) versus 26.9 +/- 4.3% (acid) in cystic fibrosis patients (n = 9). The correlations between fecal fat content measured chemically and steatocrit results were 0.18 (p = 0.35) and 0.81 (p < 0.0001) for classical and acid steatocrit, respectively. We conclude that acidification of fecal homogenates leads to a marked improvement in the steatocrit method.