Measurement of the basal metabolic rate (BMR) and the total and free serum thyroxine values in response to feeding are aids in the diagnosis of growth failure. Infants with small-for-gestational-age dwarfism gained weight poorly in the hospital, had a low BMR before and after spontaneous or induced weight gain, and a normal serum thyroxine value. Infants with linear growth failure due to chronic malnutrition had a normal BMR but a low serum thyroxine value that rose to normal with weight gain; infants with clinical signs of recent weight loss had a low BMR and a low serum thyroxine value, both of which rose to normal with weight gain. Increases of the BMR were sharp and very rapid; they preceded the rise of the serum thyroxine value in some cases.