DNA, extracted from tumours arising in 29 paediatric patients [14 neuroblastoma, 9 Wilms tumour (nephroblastoma), 6 miscellaneous] was investigated for evidence of N-myc amplification, using pNb-1, a recombinant plasmid containing a 1.0 Kb fragment homologous to the 5' end of the human N-myc gene. Within the neuroblastoma group, 4 patients had 15 or more copies of N-myc which correlated with advanced disease stage, and 3 other patients showed low grade amplification (2-5 copies). Low grade amplification was also observed in one patient with stage III unfavourable histology Wilms tumour, resistant to treatment. N-myc was present at single copy level in all other tumours studied. It is concluded that N-myc activation by amplification confers aggressive properties on a variety of embryonal tumours, rather than being restricted to initiation of neoplasia in tumours of neuroectodermal origin. A greater understanding of the complex interaction of a number of oncogenes involved in neuroblastoma may enable more effective therapeutic strategies to be devised.