Fanconi anemia (FA) is a rare autosomal recessive genetic disorder showing progressive bone marrow failure, and various phenotypic abnormalities. The lymphocytes show an increased sensitivity to the clastogenic agents diepoxybutane (DEB) or mytomycin C (MMC), measured as chromosomal aberrations. Statistical analysis of chromosome aberration yield showed that: (i) differentiation between obligate carriers and the control group was not possible; (ii) homozygotes were clearly distinguishable from heterozygotes as well as from controls by analyzing only 20 metaphase spreads per person; (iii) most of the FA patients had only one cell line present as measured by distribution of chromosomal damage among cells analyzed; (iv) and when the DEB sensitivity of a patient was high, the amount of cells without damage was low.