Neurofibromatosis 1 (NF1) disease is associated with an increased incidence of leukemias and the NF1 gene product acts as a negative regulator of the product of RAS genes which are often activated in myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML) through point mutations. Thus, we looked for abnormalities of the NF1 gene by Southern analysis in 35 cases of MDS and eight cases of AML, using cDNA probes covering the whole coding sequence. Fourteen of the patients had monosomy 17 (i.e. had lost one allele of the NF1 gene, situated in 17q11-2). Neither rearrangement nor deletion was found in any patient, suggesting that gross abnormalities of the NF1 gene must be very rare in MDS and AML. This does not exclude the possibility of more subtle abnormalities, such as point mutations, in some cases.