Gene amplification has been found in the genome of cells growing in vivo and/or in vitro. In cell lines with acquired multidrug resistance gene amplification has been frequently detected. Moreover, extra-copies of cellular oncogenes have been located in tumor cells in vivo; particularly N-myc gene amplification was discovered in advanced stage of neuroblastoma (NB). Neuroblastoma, a tumor of neural origin, has a high incidence in children. N-myc amplification has been demonstrated in untreated patient and a positive significant correlation with the progression of the disease has been established. In this paper we report on four NB patients treated with a polychemotherapeutic protocol and showing N-myc amplification. One patient examined before and after treatment displayed a slight change in N-myc gene copy numbers. It was shown that N-myc gene amplification is not affected by drug activities and that minimal residual of cells bearing N-myc amplification may remain in the tumor mass. N-myc amplification can also cause advantageous cell growth in the presence of drugs. The implications in the pharmacologic management of NB patient showing N-myc gene amplification is discussed.