Morphokinetic measurements after tetracycline labeling were performed on bone specimens of the iliac crest in a 30-year-old man and a 38-year-old women. On radiographs, the osteopetrosis (=marble bone disease Albers-Schönberg) was characterized by an almost homogenous osteosclerosis in the man and by variable zones of high and normal density in the woman. Histologically, the man revealed increased rates of bone formation and mineralisation as signs of hyperactivity of the osteoblasts accompanied, biochemically, by an elevation of serum alkaline phosphatase and a high urinary output of hydroxyproline. In the woman increased resorbing bone seams, and elevation of serum acid phosphatase and a slightly lowered urinary output of hydroxyproline were apparent as signs of an osteoclastic deficiency. The cause of osteopetrosis has been generally ascribed to decreased bone resorption. This study shows that the accumulation of bone mass in osteopetrosis may be due also to increased bone formation and that by pathomechanism, probably, two forms of osteopetrosis in the adult may be differentiated.