We performed a comparative genomic hybridization study on 25 lung adenocarcinoma samples from younger patients (<41 y of age) and compared the results with a previous comparative genomic hybridization analysis of lung adenocarcinoma samples from older patients (50-81 y of age). Twenty of the 25 tumor samples from younger patients had DNA copy number changes. Gains, losses, and high-level amplifications were seen more frequently in the specimens from the younger group. The most striking difference between the two groups was the high frequency of gains and/or high-level amplifications in the long arm of chromosome 20 in the samples from the younger patients (14/25, 56%) compared with that in the samples from the older patients (2/24, 8%, P <.001). Gains in the long arm of chromosome 22 and of the chromosomal band 11q13 were also detected significantly more often in the younger group. No correlation was found between DNA copy number changes and clinical parameters. Our results suggest that amplification of genes in the long arm of chromosome 20 may be important in the tumorigenesis of lung adenocarcinoma in young adults. Several candidate genes have already been described in the long arm of chromosome 20, particularly in breast cancer.