Identifying changes in DNA copy number can pinpoint genes that may be involved in tumor development. Here we have defined the smallest overlapping regions of imbalance (SORI) in testicular germ cell tumors other than the 12p region, which has been previously investigated. Definition of the regions was achieved through comparative genomic hybridization (CGH) analysis of a 4559 cDNA clone microarray. A total of 14 SORI were identified, which involved at least five of the 11 samples analysed. Many of these refined regions were previously reported using chromosomal or allelic imbalance studies. The SORI included gain of material from the regions 4q12, 17q21.3, 22q11.23 and Xq22, and loss from 5q33, 11q12.1, 16q22.3 and 22q11. Comparison with parallel chromosomal CGH data supported involvement of most regions. The various SORI span between one and 20 genes and highlight potential oncogenes/tumor suppressor genes to be investigated further.