The presented case is a boy with T-cell acute lymphoblastic leukemia (ALL) with hairy cell (HC) features and monoclonal gammopathy, The disease process had an acute onset and followed a rapid, progressive course. The patient had minimal splenomegaly and bicytopenia, but the bone marrow displayed increased numbers of reticulin fibers. The blasts were positive for tartrate-resistant acid phosphatase (TRAP) and CD11c, Molecular analysis revealed rearrangement of immunoglobulin heavy chain genes and a rearranged T-cell receptor (TcR beta) beta gene. The patient responded to conventional ALL therapy. Acute T-cell ALL with HC features in childhood has not been reported previously, either alone or in association with monoclonal gammopathy, We propose "T-ALL with hairy cell features" to describe this case. (C) 2000 Wiley-Liss, Inc.