<p><b>OBJECTIVE</b>To explore the clinical value of detecting adenosine triphosphate (ATP) level in CD4T lymphocytes (Immuknow ATP) of patients on early stage after allogeneic hematopoietic stem cell transplantation (allo-HSCT).</p><p><b>METHODS</b>The base-line ATP value in CD4T lymphocytes in cases of hematological malignancies and the ATP level in CD4T lymphocytes of acute leukemia patients before allo-HSCT were detected. Allo-HSCT recipients were devided into 3 groups with different level of immunereactivity according to ATP concentraiton in month 3 (day 90±5) after allo-HSCT. The clinical characteristics of patients in 3 groups were analyzed.</p><p><b>RESULTS</b>The mass concentration of Immuknow ATP in 15 cases of hematological malignancies before allo-HSCT ranged from 56.21-435.71 ng/ml, with a mean of 203.98±112.72 ng/ml. The ATP level in 46 cases after allo-HSCT ranged from 1.69-333.09 ng/ml, with a median of 41.96 ng/ml. Both 91.26 ng/ml (mean-SD) and 316.70 ng/ml (mean+SD) were used as cutoff, and 36 allo-HSCT recipients (78.3%) were assigned to low immunereactivity group, 8 recipients (17.4%) to middle group and 2 recipients (4.3%) to high group. The incidence of infection in low immunereactivity group was significantly higher than that in middle immunereactivity group (86.1% vs 50.0%)(P=0.022), and also significantly higher than that in high immunereactivity group (86.1% vs 0%)(P=0.002). There were no statistical differences in the incidences of severe infection among 3 groups. The incidence of grade II or higher acute graft versus host disease (aGVHD) in high immunereactivity group was superior to that in low immunereactivity group statistically (100% vs 13.9%)(P=0.002). Immune-mediated organ injury occurred more frequently in high immunereactivity group as compared with low and middle immunereactivity groups (100% vs 0% and vs 0%)(P=0.000; P=0.002). There were no significant differences in relapse rates of leukemia among 3 groups. The percentage of patients with increased trough blood concentration of cyclosporine A(CsA) was not significantly different among 3 groups (P=0.720).</p><p><b>CONCLUSION</b>Detection of ATP level in CD4T lymphocytes on early stage after allo-HSCT possesses clinical significance for predicting infection, severity at aGVHD and immune-mediated organ injury.</p>