It is difficult, indeed impossible, as will be shown later, to state categorically the incidence of rickets. In large cities this has been variously estimated at from 50 to 75 per cent, or more among artificially fed infants. In the course of caring for a large group of young children in a model institution, situated in the outskirts of the city, it occurred to us some years ago to inquire how frequently rickets developed under these favorable conditions. It seemed especially worth while to ascertain, by careful and continued observation, whether this disorder came about with the same frequency among these infants as among the infants in the average city home. Our group has comprised about 250 babies less than 18 months of age, who have been cared for during the past four years. A group of this kind, living under similar hygienic surroundings, and receiving at all times a