Electron microscopic examination of bone biopsies obtained from two patients suffering from pycnodysostosis revealed that osteoclasts contained (sometimes large) cytoplasmic vacuoles filled with bone collagen fibrils. These vacuoles stained positive for acid phosphatase activity, thereby suggesting that bone matrix had been phagocytosed and subsequently exposed to hydrolytic enzymes of the lysosomal apparatus. Collagen-containing vacuoles were not observed in osteoclasts of individuals not suffering from this disease.