Unremarkable medical history

A 51-year old man with unremarkable past medical history was found to have, on the occasion of a routine medical examination, a white blood cell count of 18,000 per microliter. The haemoglobin level was 14.2 g/dL, and the platelet count 210,000 per microliter. The physical examination was normal with no peripheral lymphadenopathy, spleen or liver enlargement being noted. The differential white blood cell count revealed 80% mature-looking lymphocytes. The bone marrow examination revealed 75% lymphocytes and a non-diffuse histopathological pattern. Biochemical parameters, including serum lactate dehydrogenase and β2-microglobulin, were within normal values. Discuss your diagnosis.