407 newborns with idiopathic transitory hyperbilirubinaemia were examined with regard to the decrease in serum bilirubin levels during 24 hours of intermittent phototherapy (12 hours of light exposure). The photoeffect (i.e. decrease of serum bilirubin concentration after 24 hours of therapy) showed a unique and predictable nonlinear correlation with the plasma bilirubin concentration before treatment. This relationship can be used for individualizing the duration of phototherapy and the dose of light. The apparent effect of birth weight, gestational age, and postnatal age on the efficiency of phototreatment is only due to differing initial levels of bilirubin concentration. Intermittent illumination seemed to be more efficient than continuous.