Allogeneic blood and marrow transplantation (BMT) is curative for many patients with high-risk and relapsed acute lymphoblastic leukemia (ALL). However, relapse is an important cause of post-transplantation failure, and there are no reliable markers to predict relapse. A retrospective review of patients with ALL who underwent matched related allogeneic BMT was carried out to examine whether the rate of lymphocyte recovery after transplantation had any prognostic value in ALL. The absolute lymphocyte count (ALC) at days 21 and 30 after transplantation was obtained for 43 patients who received transplants during an 18-year period. Patients with an ALC of 175 x 10(6)/l or less on day 21 were more likely to relapse than those with ALC greater than 175 x 10(6)/l (relative risk, 4; 95% confidence interval, 1.5-11.2). Patients with slower lymphocyte recovery had significantly lower relapse-free survival than those with faster recovery (P=0.0028). There was also a trend toward poorer overall survival among those with a slow lymphocyte recovery (log-rank test; P=0.028). The rate of lymphocyte recovery is prognostic in patients with ALL undergoing allogeneic BMT, and this should be integrated with other predictors to identify patients at high risk of relapse. Such patients could be considered for interventions aimed at prevention of relapse, including rapid withdrawal of immunosuppressive medication or donor lymphocyte infusion.