Investigation of the kindred of a 58-year-old woman with all of the features of "adult" hypophosphatasia revealed 12 individuals in 3 generations with subnormal circulating total alkaline phosphatase (AP) activity. The pattern of inheritance suggested autosomal dominant transmission, with incomplete penetrance of the trait particularly in the young males. Hypophosphatasic individuals other than the proposita were clinically well but had loss of permanent teeth, showing that dental abnormalities could be the only clinical manifestation of the disorder. Radiographic investigation of the proposita revealed that completion of stress fractures was necessary for healing; maturation of incomplete fractures resulted in stable Looser zones. Skeletal survey and radionuclide bone imaging were unremarkable in hypophosphatasic individuals without fracture. Subclinical osteopenia was found in several affected women by metacarpal cortical width and bone densitometric measurements. Laboratory studies showed increased plasma and urinary phosphoethanolamine levels in affected individuals. Phosphoethanolamine and phosphoserine appeared to be natural subtrates for AP since a negative correlation existed between each substrate and circulating total AP activity. Phosphoethanolamine and phosphoserine levels were greatest in the clinically affected proposita; furthermore, only she showed absence of leukocyte AP activity. Heat fractionation of her total circulating AP activity suggested severe reduction in the bone isoenzyme. Hypophosphatasic children had higher levels of total circulating AP than affected adults; the increase was apparently secondary to increased bone isoenzyme. Iliac crest bone biopsies showed greater abnormality in affected women. Osteoidosis was particularly pronounced in the proposita's younger affected sister and hypophosphatasic daughter. Histomorphometric analyses of the biopsies revealed a paucity of osteoblasts despite increased quantities of unmineralized matrix. The finding that hypophosphatasic children in this kindred had higher circulating total AP activity than adults and were able to model their skeleton normally, together with observations that the bone biopsy in adults had a paucity of osteoblasts, suggests that some factor(s) during growth is able to induce both AP activity and osteoblast function, or, that this disorder is an "abiotrophy" with deficient osteoblastic formation and/or accelerated destruction in adult life.