R elative deficiency of intestinal lactase activity during adulthood, adult hypolactasia, is a common condition worldwide. We studied the regulation of lactase-phlorizin hydrolase in normal and adult hypolactasic subjects by correlating transcript abundance in intestinal biopsies with relative synthetic rates for the protein in cultured intestinal explants. After metabolic labelling studies in six subjects, precursor lactase-phlorizin hydrolase was identified in amounts directly proportional to the enzyme-specific activity suggesting that levels of intestinal lactase are regulated by synthetic rate. Total intestinal RNA was extracted from biopsies of these subjects and three hypolactasic adults who had participated in previous biosynthesis studies. Transcript levels were markedly reduced in deficient subjects who demonstrated diminished lactase-phlorizin hydrolase synthesis. The sequence of 1 kb of 5'-flanking region of the lactase-phlorizin hydrolase gene was determined in two hypolactasic subjects and two controls. No sequence variability was identified to account for differences in mRNA levels or biosynthetic rates between the two groups. A single hypolactasic subject previously characterized as demonstrating delayed posttranslational processing, showed message levels intermediate between other deficients and controls. These results suggest that in the majority of our subjects, pretranslational mechanisms account for the predominate regulatory control of lactase-phlorizin hydrolase expression in the proximal intestine.