We describe a patient with the so-called "prolymphocytic variant" form of hairy cell leukemia (HCL) resistant to treatment with interferon-alpha (IFN-alpha). Analysis of immunoglobulin (Ig) and T-cell receptor-beta (TCR beta) gene rearrangements from serial peripheral blood mononuclear cell specimens (MNCs) confirmed not only the B-cell nature of the disease, but also the subsequent emergence of a morphologically indistinguishable population of cells with a clonal TCR beta rearrangement in addition to the original Ig gene rearrangement. With the exception of a transient increase in peripheral blood T cells during treatment with deoxycoformycin (DCF), the MNCs remained essentially constant throughout therapy with no evidence of a co-existing T-cell clone to account for the TCR beta rearrangement. Although MNCs from this patient bound significantly less IFN-alpha than did MNCs from other HCL patients, the binding was of high affinity with a kd similar to that of control cells. The number of IFN-gamma receptors on our patient's MNCs was four times higher than the number of IFN-alpha receptors and was similar to the number of IFN-alpha receptors on MNCs from HCL patients responsive to IFN-alpha. While various treatments including IFN-alpha, DCF, chlorambucil, splenectomy, leukopheresis, and IFN-gamma were not able to change the clinical progression of the disease, they may have provided an opportunity for the divergent TCR beta rearranged clone to expand and displace the initially dominant clone.