With use of a liquid-conditioned coverall, the thermoregulatory responses to a lowering of environmental temperature from 35 degrees C to 23 degrees C were assessed in eight normally nourished and six undernourished elderly female patients, during their convalescence after surgical repair of a fracture of the femoral neck. There was no difference in the peripheral vasoconstriction of the two groups in response to a cold environment. On lowering the environmental temperature, the increase in metabolic rate was significantly impaired in the undernourished group compared with the normally nourished group (P less than 0.05). There was a small decrease in core temperature in the undernourished group (median change -0.1 degree C) during the period of exposure to the lowest environmental temperature (23 degrees C). This was significantly different from the lack of change (median change 0 degrees C) in core temperature observed in the normally nourished group (P less than 0.05). This defect of thermogenesis may underlie the propensity of undernourished elderly patients to suffer hypothermia and fracture of the femoral neck in the winter months.