Chromosome I abnormalities are indicators of prognosis in neuroblastoma (NB) but are not yet routinely exploited because conventional methods are technically demanding. We evaluated the pertinence of interphase cytogenetics fluorescence in situ hybridization (FISH) for the analysis of chromosome I in NB, compared with conventional methods. Deletion of Ip was detected in 8 of 9 cell lines analyzed by both FISH and restriction fragment length polymorphism (RFLP), but was evidenced in only 2 cases by conventional cytogenetics, painting analysis being required to reveal the other cases. The chromosome I number evaluated by FISH reflected the total chromosome modal number obtained by cytogenetics. Twenty-eight specimens obtained from ultrasound-guided punctures, surgical biopsies of the primary tumor and bone-marrow aspirates were studied by FISH on frozen cytocentrifuged smears; 12 had a chromosome I trisomy and 16 a disomy. Requirements for a reliable control analysis of Ip deletion by RFLP were met in only 23 cases. The retention of 2 alleles was observed in 15 cases and Ip deletion in 7, by both techniques. In one case, an interstitial deletion of Ip was evidenced only by RFLP, and one of 5 cases analyzed only by FISH had a Ip deletion. Although FISH might be improved by using additional probes, it presents major advantages for routine exploitation. Determining Ip deletion in individual cells makes it possible to analyze small and heterogeneous tumoral specimens; the technique requires only a few hours and can easily be standardized in non-specialized laboratories. The number of chromosome I homologues per cell might serve as a rapid screening for ploidy.