We report here on the diagnosis and successful treatment of a case of hairy cell leukemia (HCL) that arose 15 years after kidney transplantation in a 51-year-old patient. As soon as the diagnosis was made, HCL was treated with 2-CDA, obtaining complete hematological remission. Immunosuppression with the calcineurin inhibitor cyclosporin was maintained, and the graft was preserved. In kidney transplant recipients supported with immunosuppressive drugs, post-transplant lymphoproliferative diseases (PTLDs) are frequent and typically related to immunosuppression via a loss of control of infectious/EBV-related proliferative stimuli. To date, HCL has not been considered among PTLDs. Recently, however, the oncogenic mutation V600E of the BRAF protein kinase has been found to be a hallmark of HCL, and calcineurin inhibitors have been shown to interfere with signaling downstream of V600E BRAF early on by counteracting senescence-associated mechanisms that protect against the oncogenic potential of the mutated kinase. Such a biochemical link between the oncogene-dependent signaling and calcineurin inhibitor activities suggests that HCL in transplanted patients might be a peculiar type of PTLD based on the presence of a specific mutation. This mechanism might also be involved in other neoplasias bearing the same or similar mutations, such as melanoma and non-melanoma skin cancer.