The mortality rate among infants who slept in a room with no other person than their parents was much lower than among those who slept in a room with more than two persons. The babies that slept in separate beds also had a much lower infant mortality rate than those who did not sleep alone, as shown in the next table. (Table omitted.) In presenting statistics on sleeping and ventilation, only the babies who lived at least one month have been considered, for the reason that so many deaths during the first month of life were due to prenatal causes. The incidence shown in the foregoing table is significant, even though it can by no means be deduced therefrom that the health of a large proportion of babies was so impaired by sleeping with older and more or less unhealthy persons that death resulted. But irregular night feeding and overfeeding are undoubtedly harmful, and the mother is tempted to subject the baby to this when it sleeps with her and disturbs her rest. Of the 1,389 babies who lived at least one month, 600, or 43.2 per cent., lived in homes where the baby slept in a room with not more than two other persons. The fact that the baby slept in a room with no more persons than its parents generally argues that the family’s means permitted them to have one or more additional rooms for other members of the family, but in other cases, of course, merely that there were no other persons in the family. Almost every home visited had means for good ventilation of the baby’s room at night, yet but 604, or 43.5 per cent., of the 1,389 babies who lived at least a month slept at night in well-ventilated rooms—that is, in rooms where, according to the mother’s statement, a window was open all night. Some mothers opened windows when the weather was neither cold nor damp; or opened them in a hall or room adjoining that where the baby slept; others emphatically stated that at night the windows were “always shut tight.” The babies subjected to differences of ventilation show corresponding variations in infant mortality rates. A high death rate in badly ventilated homes can not be charged wholly to bad air. The mother who did not, or could not, provide proper ventilation was generally the mother without the means or the knowledge necessary to enable her to care for her baby properly in other respects, and yet the marked differences suggest that ventilation is itself a very important ally of the baby in its first year of struggle for existence. In many rooms that were poorly ventilated, windows were not opened for the reason that the room was not properly heated and the houses themselves were flimsy and drafty. The problem in such houses is to keep warm. If the windows were frequently or constantly opened, the houses would be too cold to live in. In some localities the outside air is so laden with soot, ashes, dirt, and smoke that every effort is made to keep it out of the house.