The myelodysplastic syndromes (MDS) are morphologically and genetically heterogeneous, and as such a single etiological factor is implausible. Therapy-related MDS has a clear etiology but the predisposition factors remain unclear. Most MDS (>90%) is not therapy-related and an etiology for this majority of patients, and indeed of better defined (morphological or genetic) subgroups cannot yet be ascertained. Exposure to occupational and environmental toxins is not obviously a major etiological contributor. The exceptions may be exposure to low concentrations of benzene and to tobacco smoke (which contains benzene amongst other carcinogens), but even these xenobiotics produce only modestly increased Hazard ratios for the development of MDS. It seems likely that low penetrance genetic variants may influence predisposition, and these may include pathways for xenobiotic metabolism, DNA repair and other quantitative trait loci.