AIM: To measure the variation in sleeping metabolic rate (SMR) of healthy infants in the first year of life. METHODS: Attempts were made to measure SMR in the home of one group of infants at monthly intervals over the first year of life and of another group in the 1 to 12 age range, over three consecutive days. Sixty one infants were recruited and 181 measurements made. In 11 infants six or more satisfactory measurements were made a monthly intervals and in another 11 infants, three consecutive daily measurements were obtained. Parents chose the clothing and bedding that they judged their infant needed to fall asleep. The room air and radiative temperatures, humidity, and amount of insulation were recorded. RESULTS: The mean (SD) SMR was 2.4 (0.4) watts (W)/kg or 45 (10) W/m2. The rate was the same for infants in the age groups 0.25, 0.5, 0.75, and 1 years; individual infants did not show a systematic change with age. There was wide variation. Ninety eight per cent of infants had a SMR between 1.8 and 3.1 W/kg. The mean SMR for the infants measured on three consecutive days varied from 1.7 to 3.1 W/kg, and SMR varied within infants, from 0.06 to 0.86 W/kg. The variation within six to 11 measurements on infants over the first year of life ranged from 0.38 to 1.05 W/kg. CONCLUSION: Infants in the first year of life exhibit wide variations in SMR from day to day and from month to month. As the range is wide, guidance on clothing and bedding for thermal comfort can be given only in broad terms.