Chronic myelogenous leukemia (CML) is a disorder of the hematopoietic stem cell that results in malignant expansion of myeloid cells with a cytogenetic abnormality, the translocation between chromosomes 9 and 22 known as the Philadelphia chromosome. Treatment with IFN-alpha has proven to be an effective therapy, inducing cytogenetic remission in CML patients. However, it is unknown whether IFN-alpha can restore normal immune function for patients who achieve a complete cytogenetic remission. To address this question, we used a method of intracellular staining and flow cytometric analysis to ascribe the syntheses of Th1 or Th2 cytokines to T-cell subsets of patients in chronic, in accelerated, and in blast crisis phases as well as patients who had achieved a complete cytogenetic remission with IFN-alpha. We assessed the cytoplasmic synthesis of cytokine in phorbol ester (phorbol 12-myristate 13-acetate)-activated CD4+ and CD8+ T-cell subsets of 81 patients with various stages of CML and 21 normal controls. The percentages of CD4+ and CD8+ T cells from patients in chronic, in accelerated, and in blast crisis phases that synthesized Th1 cytokines interleukin (IL)-2, IFN-gamma, and tumor necrosis factor-alpha were significantly lower than those of remission patients and normal controls. Conversely, the percentages of CD4+ and CD8+ T cells of patients in chronic, in accelerated, and in blast crisis phases of CML preferentially synthesized the Th2 cytokine IL-10. Patients who achieved a durable complete cytogenetic remission for >2 years without maintenance IFN-alpha therapy restored their preference for a Th1 cytokine profile that is necessary for efficient cytotoxic T-cell function.