The myelodysplastic syndromes (MDS) are heterogeneous group of clonal disorders of hematopoietic stem cells, which manifestation is cytopenia and hypercellular, dysplastic bone marrow, often with increased amount of blasts. The pathogenesis of majority of MDS remains unexplained. It is regarded that genetic predisposition and exposure to toxic environmental agents contribute to genetic mutations in MDS. Molecular abnormalities have attracted interest over past years because of their presence in most cases of MDS, even without noticeable disorders in kariotype. Familial incidence of myelodysplastic syndromes may be helpful in understanding genetic factors of the disease. We describe two families in which MDS occurred in siblings (3 brothers; sister and brother). Kariotype abnormalities have been noted only in one person of each family. The rest of patients had a normal kariotype. This suggests that molecular abnormalities are the cause of their disease. The occupational exposure to precise mutagens (aluminium, greases, diesels, petrol, metals) was noted in two persons in the first family and correspondingly in one person in the second. There was absence of one mutagen common for every member of two reported families.