Two children are described who presented at the age of 5 and 7 months with anaemia, a high white cell count with eosinophilia and thrombocytopenia. Both children had an identical balanced translocation t(1;5)(q23;q33) and no evidence of a constitutional abnormality. The response to treatment of one child was poor, the other remains well on therapy. This translocation has not been previously reported and is likely to represent a subclass of myeloproliferative disorder analogous to the monosomy 7 syndrome, although less common. The previous literature of acquired chromosome abnormalities involving chromosomes 1 and 5 is reviewed.