We diagnosed two boys with two different chromosomal instability disorders such as Fanconi anemia (FA) and ataxia-telangiectasia (AT) in the same family. The phenotype of first sibling supports the diagnosis of ataxia-telangiectasia. He had ataxia, telangiectasias on bulbar conjunktivas, a high level of alpha-fetoprotein, low levels of Ig A and Ig E, and a defective cell-mediated immunity. Cytogenetic studies of the peripheral lymphocytes releaved a chromosomal sensitivity to ionizing radiation. His 8-year-old brother had pancytopenia but had no ataxia and telangiectasia. He had a normal level of immunoglobulins and alpha-fetoprotein. His cell-mediated immunity was also normal. Cytogenetic studies showed no evidence spontaneus chromosome aberrations; however, there were a mild increase in the rate of diepoxybutane (DEB) and also an increased chromosome aberrations in the mitomycin C (MMC) treated samples than the control. The parent of boys and 51, child were healty. The first child had a normal hematological and immunological features, but he had a mild increase in the rate of DEB. The 4(th) child had an increased rate of DEB-induced chromosome aberrations.