A 39-year-old man with a history of type 1 diabetes, presented with right hip pain on rotation, of approximately 5 years duration. No significant findings were observed on physical examination. A full blood count and peripheral blood film were unremarkable. Magnetic resonance imaging (MRI) revealed an ovoid, 4 9 3 9 2 cm, hyperintense lesion in the right sacral ala on fluid-sensitive sequences (left). The lesion was confined to the bone on MRI, and there was no apparent cortical destruction on MRI or a subsequent computerized tomography (CT) scan. A CT-guided core biopsy demonstrated sheets of small to intermediate atypical lymphoid cells with irregular nuclei, indistinct nucleoli, distinct cell borders and ample cytoplasm varying from clear to granular (right). Only rare mitotic and apoptotic figures were seen. These atypical cells were positive for CD20, PAX5, BCL2, TRAP, CD72 (DBA44), CD25, CCND1 and kappa light chain, and negative for CD10, CD5, CD117, CD138, CD56, CD43, myeloperoxidase and lambda light chain. No evidence of CCND1/IGH fusion was detected by FISH, essentially ruling out the possibility of mantle cell lymphoma. A BRAF V600E mutation was detected by polymerase chain reaction. Based on the overall morphological, immunophenotypic and molecular features, a diagnosis of hairy cell leukaemia was made. A subsequent staging bone marrow biopsy was performed and showed no evidence of involvement by a lymphoproliferative disorder. The spleen was not visualized on imaging of the pelvis but was not palpable. Treatment was initiated in the form of systemic chemotherapy and the patient commenced a 7-d course of continuous cladribine 0 1 mg daily without adverse effects. He achieved and maintained radiographic remission throughout a 6-month followup period. Very rarely, hairy cell leukaemia presents as an isolated mass (lymphomatous form) without apparent bone marrow, blood or spleen involvement. This unusual form of hairy cell leukaemia expresses the distinctive immunohistochemical and genetic hallmarks of classical hairy cell leukaemia, and is a plausible consideration in the differential diagnosis of B-cell neoplasms presenting as a mass. The effectiveness of cladribine therapy in these cases suggests that this variant is comparable to hairy cell leukaemia presenting classically.