We screened 26 ependymomas in 22 patients (7 WHO grade I, myxopapillary, myE; 6 WHO grade II, E; 13 WHO grade III, anaplastic, aE) using comparative genomic hybridization (CGH) and fluorescence in situ hybridization (FISH). 25 out of 26 tumors showed chromosomal imbalances on CGH analysis. The chromosomal region most frequently affected by losses of genomic material clustered on 13q (9/26). 6/7 myE showed a loss on 13q14-q31. Other chromosomes affected by genomic losses were 6q (5/26), 4q (5/26), 10 (5/26), and 2q (4/26). The most consistent chromosomal abnormality in ependymomas so far reported, is monosomy 22 or structural abnormality 22q, identified in approximately one third of Giemsabanded cases with abnormal karyotypes. Using FISH, loss or monosomy 22q was detected in small subpopulations of tumor cells in 36% of cases. The most frequent gains involved chromosome arms 17 (8/26), 9q (7/26), 20q (7/26), and 22q (6/26). Gains on 1q were found exclusively in pediatric ependymomas (5/10). Using FISH, MYCN proto-oncogene DNA amplifications mapped to 2p23-p24 were found in 2 spinal ependymomas of adults. On average, myE demonstrated 9.14, E 5.33, and aE 1.77 gains and/or losses on different chromosomes per tumor using CGH. Thus, and quite paradoxically, in ependymomas, a high frequency of imbalanced chromosomal regions as revealed by CGH does not indicate a high WHO grade of the tumor but is more frequent in grade I tumors.