A modified comparative genomic hybridization (mCGH) technique was applied to a series of 17 primary breast carcinomas in which cytogenetic study (CG) demonstrated the presence of homogeneously staining region(s), suggesting the occurrence of DNA amplification. mCGH demonstrated recurrent amplifications of the whole chromosome arms 8q (9 times) and 1q (7 times) and of DNA loci in the following bands: 11q13 (6 times), 9p13 and 17q21.1 (4 times), 1q21.1 and 16p11.2 (3 times), and 8q22, 8q24.1, 10q22, 15q26, 17q23, and 20q13.3 (twice). Amplification of whole chromosome arms is likely to have resulted from unbalanced translocations or isochromosomes, whereas amplifications of smaller chromosomal segments probably arose through real DNA amplification processes. In all tumors but one, more than one amplified locus was detected. The fact that many chromosomal sites were involved suggests that the process of amplification is complex and that many genes are potential targets.