We describe the clinical and biochemical features of an infant with marked transient porphyrinemia in whom blistering developed while the infant was undergoing phototherapy for severe Rh isoimmunization. The cause of the transient porphyrinemia was likely to be multifactorial--abnormal porphyrin metabolism or accumulation in a premature infant with multisystem disease and multiple drug therapy. In addition, the infant received an unusually large amount of phototherapy. No evidence for an associated porphyria has been obtained. We believe this is a unique case because transient porphyrinemia associated with neonatal blistering does not appear to have been reported previously. Furthermore, blistering associated with phototherapy is rare.