The term micropetrosis has been coined here to describe a bone condition in which the canaliculae and to a lesser extent the osteocyte lacunae are filled in vivo with mineralized tissue. This condition was found primarily in extra-Haversian bone and appeared to be a characteristic of aging. Almost no micropetrosis exists at birth, whereas an average of 15 per cent of the volume of nineteen bones from sixteen patients over seventy was micropetrotic. In three cases 40 per cent of the volume was affected.