Osteoporosis develops in White Leghorn chickens during the first year of egg production and becomes severe during the molt. Immature, and young mature pullets, cockerels, and nonmolting roosters do not have osteoporosis. The disorder appears to be a derangement of the normal process of haversian remodelling: the compact bone is resorbed without osteoclast formation by tunneling operations of blood vessels expanding inside the compact bone; new osteones are not rebuilt to replace old ones. Some of the endocrine organ changes associated with avian osteoporosis are: atrophy of the adrenal cortex and decrease in size and weight of the gonads; reductions in the weight of the thyroids and parathyroids also occur but the decline is relatively slight and inconstant. The level of calcium in the blood plasma is low and either normal or low in ultra-filtrates of serum. Exogenous estrogen (without androgen) increases resorption of cortical bone, accentuates osteoporosis, and may cause fractures from such minor trauma as excitement or handling of the birds.