Current urinary mass screening for catecholamine metabolites at 6 months of age may be detecting only a small portion of high-risk neuroblastomas: a chromosome and N-myc amplification study. We studied 96 infants and children with untreated neuroblastomas. Chromosomes of tumor cells were analyzed in 68, and N-myc copy numbers were determined in 67 patients. Patients found by a mass screening program for 6-month-old infants (group A1, 39 patients) or those less than 12 months of age found clinically (group A2, 13 patients) were rarely in the disseminated stage (A1, three of 39; A2 one of 13); their tumors usually had near-triploid (3n) or hypertetraploid (greater than 4n) karyotypes (A1, 28 of 37; A2, nine of 11), and never had N-myc amplification (A1, zero of 34; A2, zero of 11). In contrast, children 12 months or over (group B, 27 patients) were usually in the disseminated stage (19 of 27) (P less than .0001); their tumors usually had near-diploid (2n) or near-tetraploid (4n) karyotypes (16 of 20) (P = .0027), and often had N-myc amplification (nine of 22) (P less than .0001). Of the 40 clinically found patients (A2 and B), six had undergone the screening with a negative result at the age of 6 months. Two of the six patients had N-myc amplification in the tumors. Most tumors found by the screening showed known characteristics predicting a good prognosis, and the majority of tumors showing characteristics predicting a poor prognosis were found in patients aged between 12 and 36 months. Our chromosome and N-myc amplification studies suggest that a low-risk tumor does not usually evolve to a high-risk tumor. Thus, the current mass screening program may be detecting only a small portion of highly malignant neuroblastomas at the earliest stage. Infants should be screened twice, at 6 months as well as at 12 months of age, for the early detection of high-risk neuroblastomas. 