Background: Fanconi anemia (FA) is a rare autosomal recessive disorder characterized by short stature, skeletal anomalies, increased incidence of solid tumors and leukemia, and bone marrow failure (aplastic anemia). FA has been reported in all races and ethnic groups and affects men and women in an equal proportion. The frequency of FA has been estimated at approximately 1 per 360,000 live births. In some populations, including Ashkenazi Jews, Turks, Saudi Arabians and Iranians, this frequency appears to be higher, probably as a result of the founder effect and consanguineous marriage. Because of extensive genetic and clinical heterogeneity (the age of onset, clinical manifestations and survival), diagnosis of FA on the basis of clinical data alone is unreliable and its molecular diagnosis is difficult. The diagnosis of FA exploits the hypersensitivity of FA lymphocytes and fibroblasts to bifunctional alkylating agents such as mitomycin C (MMC), diepoxybutane (DEB) and nitrogen mustard and differentiates it from idiopathic aplastic anemia. In this study, in addition to the patients' clinical profiles, a cytogenetic test using MMC was implemented for an accurate diagnosis of Fanconi anemia.Methods: In this study, the lymphocytes of 20 patients referred for FA, and those of their normal sex-matched controls, were treated with three different concentrations of mitomycin C (20, 30, 40 ng/ml). Slides were prepared and solid stained. In order to determine the number and kind of chromosome abnormalities, 50 metaphase spreads from each culture were analyzed. Clinical information was obtained from patient files.Results: Five patients manifested increased chromosome breakage with MMC, confirming the FA diagnosis. Two different concentrations of MMC (30, 40 ng/ml) were most effective.Conclusion: The chromosomal breakage test is important for the accurate diagnosis of Fanconi anemia. DNA crosslinking agents used to treat idiopathic aplastic anemia may be lethal for patients with FA. Thus, aplastic anemia patients with unknown etiology, infants with congenital abnormalities involved in FA and siblings of FA patients should also be cytogenetically tested."n