Histochemical and direct enzyme analysis of osseous tissue from 23 patients with hypophosphata-sia revealed that all clinical forms of this inherited metabolic bone disease are characterized by deficiency of alkaline phosphatase (ALP) activity in bone. The severe infantile form has the most profound deficiency, yet the cellular source of this enzymeosteoblasts and their matrix vesicles are normal by routine light and electron microscopy. Despite radiographic changes in bone metaphyses consistent with rickets, iliac crest biopsy of one affected child revealed no abnormalities; the other had evidence of a mineralization defect, but not as severe as that in affected infants. In this child and several affected adults with osteomalacia, osteoblasts appeared flat and metabolically inactive. Although these histological changes suggested a different pathogenetic mechanism for adult and childhood hypophosphatasia, these changes are most likely secondary to the underlying osteomalacia. Our findings are most consistent with evidence that childhood and adult hypophosphatasia often represent clinical expression of the heterozygous state for ALP deficiency which, when homozygous, results in the clinically severe, recessive, infantile form. Histochemical and direct analysis of bone tissue from controls and patients with hypophos-phatasia demonstrated that the severe infantile form is associated with the most severe ALP deficiency. In the milder clinical forms, ALP deficiency in bone is not as profound. In general, the severity of the clinical expression of hypophosphatasia reflects the magnitude of the deficiency of ALP in bone. This is the expected finding for this inborn error of metabolism, which illustrates the major role bone ALP activity has in the process of normal skeletal mineralization.