ABSTRACT: Mycosis fungoides patients who develop tumors or extracutaneous involvement usually have a poor prognosis with no curative therapy available so far. In the present EORTC multicenter study, the genomic profile of 41 skin biopsies from tumor-stage mycosis fungoides was analyzed using a high-resolution oligo-array comparative genomic hybridization platform. Seventy-six percent of cases showed genomic aberrations. The most common imbalances were gains of 7q33.3q35 followed by 17q21.1, 8q24.21, 9q34qter and 10p14 and losses of 9p21.3 followed by 9q31.2, 17p13.1, 13q14.11, 6q21.3, 10p11.22, 16q23.2 and 16q24.3. Three specific chromosomal regions, 9p21.3, 8q24.21 and 10q26qter were defined as prognostic markers exhibiting a significant correlation with overall survival (P= .042, P= .017 and P= .022, respectively). Moreover, we have established two MFt genomic subgroups distinguishing a stable group (0-5 DNA aberrations) and an unstable group (> 5 DNA aberrations), showing that the genomic unstable group had a shorter overall survival (P=.05). We therefore conclude that specific chromosomal abnormalities, such as gains of 8q24.21 (MYC) and losses of 9p21.3 (CDKN2A, CDKN2B and MTAP), and 10q26qter (MGMT and EBF3) may play an important role in prognosis. In addition, we describe the MFt genomic instability profile. Forty-one MFt were studied by arrayCGH using the Human Genome CGH 44K microarrays (G4410B and G4426B, Agilent Technologies, Palo Alto, CA, USA). In each microarray experiment, DNA obtained from a 20x10 um sections snap frozen samples from tumoral MF lesions was compared with commercial pools of healthy female DNA (Promega, Madison, WI, USA).