Abstract: The action of light-therapy in infants with hyperbilirubinaemia has been intensively investigated, the conversion of bilirubin to the water-soluble isomer cyclobilirubin (lumirubin) has been described in infants and in a 15 year old girl with Crigler Najjar syndrome. The purpose of present study was to see if lumirubin is formed during light therapy also in adults. While control measurements exhibited a steady decrease of lumirubin fluorescence, there was a steady increase of fluorescence during light therapy. Urine samples taken during light therapy at 10 min. intervals showed a highly significant (p<0.01) increase of 415 nm fluorescence when excited at 315 nm, compared to controls under dark conditions. Bilirubin fluorescence in plasma during light therapy showed a significant decrease after 50 min. These results show that lumirubin is formed and excreted during light therapy also in adults. In the last section possible implications of these findings for the action of light on biological rhythms and mood are discussed.