We measured the changes in cutaneous bilirubin (Br) and serum Br photoisomers in two groups of 5 jaundiced newborn infants treated by intensive phototherapy (IP), one with blue light and the other with green light. Cutaneous Br was measured with a transcutaneous jaundice meter and photoisomers were measured by HPLC. Cutaneous Br decreased in the two groups as soon as IP began, and the skin was completely bleached within 3 h with blue light only. Rebound occurred which was more marked after blue light IP. The main serum photoproduct was the 4Z-15E isomer, which reached a steady-state level within 1 h in both groups, whereas the lumirubin and 4E-15Z Br concentrations were slightly higher after green light IP. These data indicate that blue light is more suitable for IP, although this is not clearly explained by the production of more lumirubin.