Lactase-phlorizin hydrolase (LPH) splits lactose in the small intestine. LPH activity is high in the suckling; in many human populations the activity declines in adults, leading to adult-type hypolactasia, whereas in other populations the high LPH activity persists in adults. In the present work, we compared LPH sequences at the gene and cDNA level among adult subjects with high and low LPH activity. The complete intron-exon organization, including the sequences of all 17 exons and of the borders of all introns (as well as about 1,000 bp of 5' flanking region), was established for the cloned chromosomal LPH gene of a subject with persistence of lactase. Using PCR, we directly sequenced the exons of a hypolactasic subject. Except for silent mutations and the unknown linkage phase at two heterozygous positions, both coding sequences were identical. We further examined the LPH mRNA of a hypolactasic subject by S1 mapping and by sequencing a set of overlapping PCR products produced from cDNA templates. Except for allelic differences, the LPH sequence of the hypolactasic subject was identical to that of the LPH cDNAs of three subjects with persistence of lactase (one cDNA isolated previously by cloning and two characterized in the present work by PCR). No allele was peculiar to the hypolactasic subject. We conclude that humans with high or low levels of lactase can code for identical LPH enzymes.