We report a patient with hairy cell leukemia Japanese variant (HCL-Jv) that developed after radiotherapy for orbital adnexal MALT lymphoma. A 78-year-old man was diagnosed as having MALT lymphoma in the left conjunctiva in December 2003. The patient was treated by local radiotherapy and the tumor disappeared. Thereafter, he gradually developed leukocytosis and mild splenomegaly. In May 2009, the leukocyte count was 34,300 with 80% lymphoid cells. A diagnosis of HCL-Jv was made since the lymphoid cells showed a hairy morphology with round nuclei and indistinct nucleoli. These cells expressed CD11c, CD19, CD20, CD103 and showed weak reaction for tartrate-resistant acid phosphatase (TRAP). Bone marrow was infiltrated by atypical cells with an intrasinusoidal pattern. No treatment was needed as the patient was asymptomatic without anemia, thrombocytopenia or lymphadenopathy. Results of the immunoglobulin light chain expression and the heavy chain rearrangement in the tumor cells indicated that the two mature B-lymphoid neoplasms, MALT lymphoma and HCL-Jv, in this patient were derived from independent clones. This appears to be the first reported case of HCL-Jv associated with other lymphoid tumor. Further analysis is needed to clarify the risk of secondary malignancy in HCL-Jv.