Forty-four malignant fibrous histiocytomas (MFHs) were studied by comparative genomic hybridization. Among the observed imbalances, losses of the 13q14-q21 region were observed in almost all tumors (78%), suggesting that a gene localized in this region could act as a tumor suppressor gene and that its inactivation could be relevant for MFH oncogenesis and/or progression. We determined by CA repeat analyses a consensus region of deletion focusing on the RB1 region. The RB1 gene was then analyzed by protein truncation test, direct sequencing, fluorescence in situ hybridization, Southern blotting, and immunohistochemistry. RB1 mutations and/or homozygous deletions were found in 7 of the 34 tumors analyzed (20%). Among the 35 tumors with comparative genomic hybridization imbalances analyzed by immunohistochemistry, 30 (86%) did not exhibit significant nuclear labeling. The high correlation between chromosome 13 losses and absence of RB1 protein expression and the mutations detected strongly suggest that RB1 gene inactivation is a pivotal event in MFH oncogenesis. Moreover, the observation of a high incidence of MFH in patients previously treated for hereditary retinoblastoma fits well this hypothesis.