The last decade has seen a dramatic improvement in prognosis of hairy cell leukaemia (HCL) but concern has emerged with regard to the incidence of second malignancy. A recent report found an 18.8% incidence of second cancers and suggested a possible role for alpha-interferon (IFN) in their pathogenesis. We reviewed our larger series of 200 patients with HCL. We found second malignancies in 8 cases (4.0%), all but one of whom had received IFN. However, when compared to age- and sex- matched population data this represents no increase in relative risk of second cancer in patients with HCL and provides no evidence of a role for IFN in the pathogenesis of these second malignancies.