N-myc gene amplification was studied in a consecutive series of neuroblastomas and ganglioneuromas, representing all of such tumours diagnosed in Sweden over a 4-year period. Both frozen and formalin fixed specimens were used for Southern blot analysis. Thirty-seven of 46 neuroblastomas and 7 of 9 ganglioneuromas were analyzed. Seven neuroblastomas and none of the ganglioneuromas showed N-myc gene amplification. All children with amplified tumours, including three infants, had advanced disease at diagnosis and aggressive course of disease. However, follow-up time was short for the two cases still alive. The use of an ultrasound guided cutting needle biopsy technique for obtaining the required tissue at diagnosis was evaluated in some cases. This technique appeared to be safe and clinically useful since early prognostic information was obtained. Using an imprint from the needle biopsy, the representativity could be confirmed. Ultrasound guided cutting needle biopsies can thus be used routinely to obtain N-myc gene amplification data prior to initiation of therapy in neuroblastoma.