AbstractBrain tumors are the second most common childhood cancer. We used high‐resolution array comparative genomic hybridization (aCGH) to analyze losses and gains of genetic material from 24 medulloblastomas. The bacterial artificial chromosome clones were ordered on the array, allowing for an average resolution of approximately 420 kilobases. The advantage of this high resolution is that the breakpoints associated with subregional chromosome copy number aberrations can be accurately defined, which in turn allows candidate genes within these regions to be readily defined. In this analysis, we confirmed the frequent involvement of loss of 17p and gain of 17q, although we have now established the position of the breakpoint that consistently lies in the chr17:18318880–19046234 region of the chromosome. Other frequent losses were seen on 8p, 10q, 16q, and 20p, and frequent gains were seen on 2p, 4p, 7, and 19. In addition, the fine‐resolution mapping provided by aCGH made it possible to define small chromosome deletions in 1q23.3–q24.2, 2q13.12–q13.2, 6q25–qter, 8p23.1, 10q25.1, and 12q13.12–q13.2. Overall, amplification events were rare, the most common involving MYC (16%), on 8q, although isolated events were seen in 10p11 and 3q. © 2005 Wiley‐Liss, Inc.