Serial skin (sole) and rectal temperatures were simultaneously taken from 55 healthy and 26 septicaemic newborn infants to find out prospectively whether septicaemic newborn infants have any thermoregulatory reaction to the septicaemia, and whether regular temperature measurements could help in the early diagnosis of septicaemia. The septicaemic infants were divided into three groups: the first comprised eight feverish infants, the second 11 with normal temperatures who were in relatively good clinical condition, and the third seven with normal temperatures who were in poor clinical condition. All 55 healthy babies had rectal temperatures of less than 37.8 degrees C and a mean rectal-sole temperature difference of 2.5 degrees C. The first group of septicaemic infants had rectal temperatures over 37.8 degrees C and a high mean temperature difference of 6.9 degrees C, whereas the second group had rectal temperatures less than 37.8 degrees C and a mean temperature difference of 4.7 degrees C. Infants of the third group had a low rectal temperature and a low mean temperature difference (1.1 degrees C). We conclude that septicaemic newborn infants show an adequate thermoregulatory reaction, which is reflected by a widening of the rectal-sole temperature difference of more than 3.5 degrees C, except for those who are critically ill, who lose this ability. In view of these results infants with normal temperatures but with a rectal-sole temperature difference of more than 3.5 degrees C should be suspected of septicaemia and investigated thoroughly.