A father and son who both developed chronic neutrophilic leukaemia (CNL) are reported. The father, aged 63, had been exposed to radioactive fallout after the atomic bomb attack on Hiroshima; he presented with hepatosplenomegaly and neutrophilic leucocytosis, and died of intracerebral haemorrhage 1 month after diagnosis. 4 years later his son, then aged 44, presented with neutrophilic leucocytosis. Leukaemic transformation to acute myelogenous leukaemia (AML-M1) occurred, and he died of refractory leukaemia 4 months after the diagnosis of CNL. This is the first report of this rare disease occurring in family members; genetic effect due to radioactive poisoning was suspected in the development of CNL in these two cases.