A neuroblastoma transformed into a ganglioneuroma was well studied by the use of large sections which allowed the observation of the actual transformation of one element into the other. Despite extensive therapy and despite the presence of large areas of maturation the child died with widespread neuroblastomatous metastases. The attention of pathologists and clinicians is again directed to the necessity of studying such cases in detail, in order to comprehend fully the factors involved in the intriguing question of maturation of neuroblastomatous tumors.