The 5qϪ syndrome is a distinct type of myelodysplastic syndrome (MDS) characterised by refractory anaemia, morphological abnormalities of megakaryocytes, and del(5q) as the sole cytogenetic abnormality. In contrast to patients with therapy-related MDS with 5q deletions, 5qϪ syndrome patients have a favourable prognosis and a low rate of transformation to acute leukaemia. We have previously delineated a common deleted region of 5.6 Mb between the gene for fibroblast growth factor acidic (FGF1) and the subunit of interleukin 12 (IL12B) in two patients with 5qϪ syndrome and small deletions, del(5)(q31q33). The present study used fluorescence in situ hybridisation (FISH) analysis of these and a third 5qϪ syndrome patient with a small deletion, del(5)(q33q34), to refine further the critical deleted region. This resulted in the narrowing of the common deleted region within 5q31.3-5q33 to approximately 3 Mb, flanked by the adrenergic receptor  2 (ADRB2) and IL12B genes. The common region of loss in these three 5qϪ syndrome patients includes the macrophage colony-stimulating factor-1 receptor (CSF1R), secreted protein, acidic, cysteine-rich (SPARC), and glutamate receptor (GRIA1) genes. This 5qϪ syndrome critical region is telomeric to and distinct from the other critical regions on 5q associated with MDS and acute myeloid leukaemia.