T-bet protein levels correlate with disease activity. Patients were re-examined for T-bet levels 3 to 4 months after they were first examined for T-bet expression. In patients whose disease status was unchanged, there were no differences in T-bet expression. In patients who had responded to treatment, T-bet levels were lower. (A) Patient no. 12, who relapsed when first studied, by 3 months had progressive pancytopenia, and T-bet levels were elevated at both time points. (B) Patient no. 15 was in remission and continued in remission 4 months later; no T-bet could be detected at either occasion. (C) Patient no. 3 was at diagnosis when there was high expression of T-bet; 4 months later, response to treatment was associated with lower levels of T-bet.