The correlation between the cephalo-pedal progress of clinical icterus and the serum bilirubin concentration was examined in newborn infants without blood type sensitization. In daylight, 374 observations were made, 290 on 121 mature infants and 84 on 24 premature infants. The investigation showed that a caudad progress of icterus corresponded with an increasing serum bilirubin concentration, and repeated observations can demonstrate whether icterus progresses, showing that the concentration is increasing. The same relationship was found in respect of all the infants with the exception of the two smallest (birth weight less than or equal to 1000 g). They were icteric on the feet at lower concentrations than in the other infants. Furthermore, the relationship did not depend on which day the observations were made. It was not necessary to determine the bilirubin concentration until icterus had progressed to below the knees, as a concentration less than or equal to 110 mg/1 corresponded to all observations in which the lower limit of icterus was found proximal to the knees. However, in the small premature infants it would have been reasonable to measure the concentration when icterus had reached the area below umbilicus.