In contrast to patients affected with Fanconi's anaemia (FA), who are homozygotes, parental heterozygotes are generally considered normal. In this preliminary work the main immunological phenotypes of circulating mononucleated cells were studied, both in homozygous and heterozygous subjects. The statistical analysis of results showed that two sets of variables may be distinguished: (a) the first set, including CD20, CD4, CD8 cell markers and CD20/CD3 and CD4/CD8 ratios, that was able to differentiate between FA patients and the other subject groups; (b) the second set, including CD25, HLA-DR, HLA-DP, HLA-DQ cell markers, that was able to differentiate healthy subjects from the other groups. Therefore, in contrast with the literature data, immunological abnormalities may already be present in parental heterozygotes of FA patients. These subjects displayed a reduced number of cells expressing both specific (CD25) and non-specific (HLA) antigens. This defect was more severe in FA homozygous patients, who showed, in addition, a reduced total lymphocyte count, reduced levels of T helper (CD4+) and B lymphocytes (CD20+), and a reduced CD4+/CD8+ cell ratio. In conclusion, our results suggest the presence of a grading of immunological defects in FA patients and family members. Our suggestion needs to be confirmed by functional studies.