2-chlorodeoxyadenosine (2-CdA) induces high complete remission (CR) rates in hairy cell leukaemia (HCL), but is associated with serious toxicities. Therefore we reviewed our experience with 2-CdA in 16 HCL patients, with special attention to adverse effects. One-third of patients presented severe neutropenic infections and/or required prolonged blood support. Patients with low tumour mass and moderate cytopenias were more likely to achieve CR, whereas those with high tumour burden and severe bone marrow impairment were at increased risk of severe infection and blood product requirements. All these unfavourable parameters may be corrected by short-term alpha-interferon (IFN) therapy. Therefore we suggest that patients with unfavourable presenting features might benefit from IFN therapy before 2-CdA.