The first association of polycythaemia rubra vera (PRV) and hairy cell leukaemia (HCL) is reported. The secretion by spleen cells in short term culture of IfGlambda to the exclusion kappa demonstrated that despite aberrant surface marker studies the HCL was a monoclonal tumour of B cells. Chromosomal studies on splenic cells were not able to demonstrate the common clonal origin of the two tumours.