Hairy-cell leukemia is a chronic B-cell malignancy seen in adults. The presenting manifestations consist of splenomegaly, pancytopenia, and characteristic monocyte depletion. The presence in peripheral blood or bone marrow of hairy cells exhibiting the CD19(+) CD20(+) CD25(+) CD11c(+) phenotype establishes the diagnosis. Rarely, patients present with inaugural joint manifestations related either to the hematological malignancy or to immune dysfunction. The resulting polymorphic polyarticular symptoms may cause diagnostic wanderings. Monocytopenia is a valuable diagnostic clue. The identification of hairy cells in the joint fluid establishes the diagnosis of leukemia-related arthritis. The treatment rests on purine analogs. One of the main differential diagnoses is Felty's syndrome, which combines rheumatoid arthritis, splenomegaly, and neutropenia. Felty's syndrome is usually caused by T-cell lymphoproliferative disorders. Among 27 patients with hairy-cell leukemia managed at our institution, 1 presented with joint manifestations. We describe this case.