Phototherapy of neonatal jaundice involves Z-E-isomerisation around an exocyclic double bond in bilirubin. Our results of a CASSCF study on dipyrrinone, a bilirubin model, show a conical intersection between the ground and first excited singlet states associated with the Z-E-isomerisation. The conical intersection, located ca. 50 kJ/mol below the Franck-Condon-point, together with the S-1 minimum, ca. 50 kJ/mol below the conical intersection, are able to explain the available time-resolved spectroscopic data (the very short lifetime of the initially excited state and transient 'dark state' intermediate) as well as bilirubin's very low fluorescence quantum yield and the medium-efficient photoisomerisation reaction. (c) 2006 Published by Elsevier B.V.