Chromosome studies have been performed in bone marrow and in venous blood cells of two patients with paroxysmal nocturnal haemoglobinuria (PNH). The frequency of aneuploidy in marrow cells was of 40 and 35%, respectively. Blood lymphocytes showed aneuploidy in only one patient.The data are consistent with the hypothesis that PNH can be a myeloproliferative disease, but no direct evidence has been found neither for a clonal composition of PNH nor for participation of all marrow cell lines