Abstract: Hairy cell leukaemia (HCL) is a rare lymphoproliferative disorder. Common mode of presentation is the sequelae of bone marrow failure and its complications. Here we report a patient with an extremely rare presentation of recurrent mono arthritis with pancytopenia which preceded months before the diagnosis of HCL, which was confirmed on a subsequent bone marrow examination. Inflammatory and degenerative aetiologies for monoarthritis were excluded. His joint related symptoms subsided on specific treatment for HCL.