Fanconi's anaemia (FA) is an autosomal recessive disorder characterized by progressive bone marrow failure and a susceptibility to cancer. Haematopoietic stem cell transplantation is the only curative method for restoring normal haematopoiesis, and survival is improved if the transplant is carried out before severe complications occur. However, the evolution of FA is difficult to predict because of the absence of known prognostic factors and the unknown function of the genes involved. In studying 71 FA patients, a correlation was found between severe aplastic anaemia (SAA) and the individual annual telomere-shortening rate (IATSR) in peripheral blood mononuclear cells (P < 10(-3)). Spontaneous apoptosis was highest in SAA patients or patients with high IATSR (> 200 bp/year) (P < 0.01, n = 18). Univariate and multivariate analyses showed that significant relative risks for evolution towards SAA were high IATSR (P < 10(-4)), and that a high number of chromosome breakages occurred in the presence of nitrogen mustard (P < 0.001). A high IATSR was also associated with an increased frequency of malignancy (P < 0.01). Thus, these biological parameters were related to the spontaneous evolution of FA and could be used as prognostic factors. These data indicated that telomeres might play a role in the evolution of bone marrow failure and malignant transformation in FA.