Hairy cell leukemia (HCL) is a lymphoproliferative malignancy characterized by bone marrow, spleen, liver, and occasionally lymph node infiltration with hairy cells, usually accompanied by splenomegaly and pancytopenia. We report an unusual case of HCL in a 53-year-old woman with leukopenia and sudden onset of fever of unknown origin, arthritis, and generalized maculopapular exanthem. Skin biopsy revealed perivascular and/or vessel wall lymphocytic infiltration in the dermis. On the basis of bone marrow biopsy and flow cytometry, the diagnosis of HCL was established. A detailed, retrospective re-evaluation of the skin biopsy helped to identify hairy cells among the cells of the perivascular infiltrations. Small-vessel vasculitis, as an atypical presentation, was found to predate a diagnosis of HCL. Recognition that vasculitis can reflect or antedate lymphoid malignancy may permit early diagnosis and aggressive treatment. A rapid response was obtained with a single course of cladribine.