Fluorescence in situ hybridization (FISH) was applied to neuroblastoma for detection of N-myc (MYCN) oncogene amplification, and the results were compared with Southern blot analysis (Southern). In nine neuroblastomas (formalin-fixed paraffin-embedded tissues were available in seven cases including two cases with touch preparations, and two cell lines), all five cases with N-myc amplification detected by Southern had cells with multiple N-myc signals by FISH, and three cases showed no N-myc amplification either by Southern or FISH procedure. One case, not examined by Southern, showed amplified signals of N-myc by FISH. These data indicate that FISH results for N-myc amplification have close correlation with Southern blot analysis. The chromosome 2-specific repetitive DNA probe was also applied for the analysis of ploidy by FISH. Six cases with N-myc amplification by Southern and/or FISH had diploid tumors and two cases without amplified N-myc showed aneuploidy. The remaining one case consisted of heterogeneous elements showing diploidy in undifferentiated tissue and both aneuploidy (ganglionic cells) and diploidy (Schwann cells) in differentiated area. We conclude that FISH is a practical, useful and reliable method over Southern especially for analysis of N-myc amplification in neuroblastoma, and simultaneous cohybridization with a specific chromosome probe is of great value in predicting the prognosis of patients.