BACKGROUND Adult lactase deficiency may result either from diminished synthesis of precursor lactase-phlorizin hydrolase (phenotype I deficiency) or from altered posttranslational processing of the precursor protein (phenotype II). The aim of this study was to compare the location of lactase-phlorizin hydrolase along the crypt-villus axis in control with adult lactase-deficient subjects. METHODS The immunocytochemical distribution of lactase-phlorizin hydrolase was studied in subjects with adult hypolactasia and in controls with persistent high lactase expression. RESULTS Duodenal biopsy samples from controls showed increasing intensity of brush border staining from upper crypt to midvillus. Subjects with phenotype I deficiency showed reduced and more patchy reaction product over brush borders. In some, maximal staining was localized over the upper half of the villi. The subject with phenotype II deficiency showed patchy staining but also intracellular accumulations of immunoreactivity within the apex of enterocytes mainly on the upper half of villi. Subsequent immunoelectronmicroscopy showed nearly a fourfold increase in label over the endoplasmic reticulum. CONCLUSIONS These findings support prior observations of diminished enzyme synthesis in phenotype I lactase deficiency and suggest that the alterations in posttranslational processing in phenotype II deficiency involve a partial block in transport from endoplasmic reticulum to Golgi apparatus.