The thermal insulation of clothing and wrapping (tog value), room temperature, and body temperature was measured for 3-4 month old infants sleeping in their home cots under conditions chosen freely by parents during a cold winter. We found that ambient temperature averaged 18.4 degrees C when infants were put down, but fell by an average of 4.4 degrees C during the night. Minimum room temperature correlated with outside temperature, but most rooms were heated to some degree; smaller babies were kept in warmer rooms. The tog value of clothing before putting the baby down averaged 5.1, supplemented by 9.6 tog units of wrapping in the cot--a 188% increase for a 4.4 degrees C drop in temperature. Total tog of clothing and wrapping correlated negatively with minimum room temperature; smaller born babies tended to be more heavily wrapped. Despite the large increase in insulation in the cot, most babies maintained normal body temperatures.