Between June 1983 and December 1990, cytogenetic analysis was performed on 82 neuroblastoma patients, including 41 patients found by mass screening. The N-myc copy number was determined in 77 of the 82 patients. Patients were classified into three groups: patients found by mass screening (group A) (n = 41), patients found clinically who were under 12 months of age (group B) (n = 12), and patients found clinically who were over 12 months of age (group C) (n = 29). All patients in group A had hyperdiploid (H-2n) or near-triploid (3n) karyotypes; they were all completely free of tumor for 4-84 months after diagnosis, although one patient relapsed and attained complete remission again. Among those in group B, one patient with a stage 4 tumor had a near-diploid Karyotype (2n) and N-myc amplification, and died 18 months after diagnosis. Another patient aged 2 days with a stage 4S tumor had near-diploid karyotype and N-myc amplification, and died 9 months after diagnosis. Tumors in the remaining 18 patients in group B had hyperdiploid or near-triploid karyotypes, they lacked N-myc amplification, and the patients were free of tumor for 12-89 months after diagnosis. All patients in group C except two have near-diploid or hypotetraploid karyotypes, and 21 of 29 patients died within 18 months. One 13-month-old patient (classified in stage 2) and another 7-year-old patient (stage 2) had near-triploid karyotypes and were alive at 74 and 72 months, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)