Axial osteomalacia-a rare osteosclerotic bone disorder characterized by axial skeleton pain, coarsening of the trabecular bone pattern on radiographs of the axial but not appendicular skeleton, and osteomalacia on biopsy of a rib or iliac crest-has been reported in 10 apparently sporadic cases, all of which were in middle-aged or elderly Caucasian men. The etiology is unknown but has been postulated to be a bone cell defect. We describe the clinical, laboratory, pathologic and family study of a black mother and son with axial osteomalacia associated with polycystic liver and kidney disease. Investigation of the son suggested that radiographic osteosclerosis can be detected in early adulthood. Limited skeletal survey of his three children revealed no abnormalities. Examination of undecalcified iliac crest bone after in vivo tetracycline labeling revealed severe osteomalacia in the son despite normal circulating calcium, inorganic phosphate and vitamin D metabolite levels and persistently elevated alkaline phosphatase activity. Although osteoblasts appeared flat and inactive, histochemical studies showed intense alkaline phosphatase activity in the osteoblasts along most trabecular bone surfaces. Electron microscopy revealed intact matrix vesicles within unmineralized osteoid. The presence also of unexplained myopathy in the son-characterized by proximal muscle weakness, persistently elevated circulating creatine phosphokinase levels and pathologic changes of myopathy on biopsy of quadriceps muscle-together with impaired bone mineralization, suggests that a disorder of vitamin D action may be involved in the pathogenesis of this unusual condition. Axial osteomalacia affects blacks as well as Caucasians, women as well as men, may be familial, and may perhaps be a developmental abnormality inherited in association with polycystic kidney and liver disease.