In two sibships, four patients with Fanconi's anaemia were studied cytogenetically. Interfamilial variations of the frequency of chromosome breakage and rearrangements were found, suggesting the heterogeneity of the disease. Variations of chromosomal abnormalities have also been found during the course of the disease. An abnormal 47 chromosomes clone has been observed in the bone marrow cells from one patient. A mechanism of somatic segregation is probably involved in the constitution of that clone.