Allogeneic BMT is potentially curative for patients with acute myelogenous leukemia (AML) in first remission. However, many patients relapse after transplantation. Various immunotherapeutic options have been attempted with variable success in preventing relapse. Early identification of patients at high risk for relapse could allow prompt intervention. We examined the effect of slow lymphocyte recovery after sibling-matched allogeneic BMT on the risk of relapse in patients with AML. We also examined the effect of prednisone-containing GVHD prophylaxis on the rate of lymphocyte recovery. Patients with absolute lymphocyte count (ALC) <150 x 10(6)/l by day +30 had a 3.5-fold higher risk of relapse (P = 0.0088) and a lower overall survival (P = 0.0079) than patients with a higher ALC. We did not find correlation between lymphocyte count determined earlier in the post-transplantation course (day +21) and the risk of relapse. Patients receiving prednisone had a significantly lower ALC at day +30 than those who did not receive prednisone (289 vs 549 x 10(6)/l, P = 0.002). We conclude that a slow lymphocyte recovery after allogeneic BMT for AML is strongly predictive of subsequent relapse and that the type of GVHD prophylaxis should be considered when analyzing lymphocyte recovery.