Hypersensitivity to mosquito bites (HMB) is known to be an allergic reaction and also a skin symptom in some cases of natural killer cell leukemia/lymphoma associated with Epstein-Barr virus (EBV) infection. We describe a patient who had suffered from HMB for 5 years, and subsequently developed mantle cell lymphoma (MCL), which resembled chronic lymphocytic leukemia in the early phase. EBV monoclonality was not detected in lymph nodes by Southern blotting. Serum levels of interleukin-4 and IgE were increased, but they decreased and HMB disappeared when MCL was predominant. We consider that HMB may be a potential sign of MCL.