The genes for acetylcholinesterase (ACHE) and butyrylcholinesterase (BCHE) are located within regions subject to non-random chromosomal abnormalities in the myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML). Acetylcholinesterase is mapped to 7q22, within the critical deleted region presumed to contain a myeloid specific tumour suppressor gene. Butyrylcholinesterase is mapped to 3q26: abnormalities at this region are associated with sub-types of MDS and AML with thrombocytopenia, or with increased platelet counts. Both ACHE and BCHE have been implicated as playing a role in megakaryopoiesis and thrombopoiesis, and these genes have been observed to be co-amplified in acute myeloid leukaemia. Recent findings suggest a more significant role for the ACHE gene in haemopoiesis by regulating multipotent stem cell proliferation, and apoptosis in cells undergoing erythroid and myeloid differentiation. This led us to investigate gene copy-number alterations at these genes in MDS and AML. Samples were screened by slot-blot hybridization, and if changes were observed, by Southern blotting. A total of 42 samples from 31 de novo AML patients, 10 samples from eight cases of post-MDS AML and 85 samples from 67 MDS patients were analysed with probes for ACHE, BCHE, c-MYC, MDR-1 and globin control. Changes in ACHE and/or BCHE were observed in 9/31 de novo AML patients, and in 7/67 MDS patients: 1/37 cases of refractory anaemia (RA), 1/10 cases of refractory anaemia with excess blasts (RAEB) and 5/20 chronic myelomonocytic leukaemia (CMML) patients. The amplification events observed generated copy numbers no greater than 10, showed normal restriction patterns and had no clear correlation with megakaryopoiesis or thrombopoiesis. Loss of signal at the ACHE locus was observed: haploid signal intensity was seen in seven samples: one RA with thrombocytopenia, three CMML, one AML-M5a (no karyotypic abnormalities of chromosome 7), one AML-M4 (monosomy 7), and one case of AML-M7 (karyotype unknown). Homozygous deletion was observed at relapse of an additional patient with AML-M4. These data reinforce the possibility that ACHE may play a role as a myeloid tumour suppressor gene.