We describe a case of hairy cell leukaemia (HCL) coexistent with non-Hodgkin's lymphoma (NHD). This combination is reported to be extremely rare with no clear demonstration of the clonal relationship between the two conditions. After a previous failure of purine analogue therapy, our patient was successfully treated with rituximab resulting in normalisation of blood cell count cessation of blood transfusion and negative iliac crest biopsy. Unfortunately, the patient developed intense and persistent bone pain during the 1(st) line treatment for HCL. Skeletal X-rays, neck-thorax-abdomen CT scan and repeated bone MRI were unremarkable and bone scintigraphy showed non-specific changes. Laboratory examinations were normal. To better evaluate bone scintigraphy results, we finally performed FDG-PET/CT, which showed multiple foci of intense abnormal radiotracer uptake involving the bone marrow. An FDG-PET/CT guided bone marrow biopsy showed primary bone marrow diffuse large B-cell lymphoma (LBCL). Despite 2(nd) and 3(rd) line treatment, the patient died shortly after for central nervous system involvement by NHD. The role of FDG-PET/CT in identifying bone and bone marrow localization of NHD is reviewed and an earlier use is suggested in poorly understood bone pain.