Flow cytometric DNA analysis was performed on 45 neuroblastomas in order to evaluate the prognostic significance of DNA ploidy. The DNA ploidy was diploid in 14 and aneuploid in 31 neuroblastomas and did not correlate with the conventional prognostic variables such as age, Evans' stage, or the primary site of the tumor. The survival of 31 patients with aneuploid tumors was comparable to that of 14 patients with diploid tumors. On the other hand, DNA aneuploidy was associated with a decreased long-term survival in 21 patients older than 24 months of age at the time of operation, and in 27 patients with advanced disease at Evans' stage III or IV. These results suggest that more intensive and longer-term postoperative chemotherapy is necessary for neuroblastoma patients older than 24 months of age, or for those with advanced disease if they have aneuploid tumors.