Carbonic anhydrase 9 (CA9) is a marker for decreased O₂ concentration and acidosis, associated with poor prognosis in cancerous patients. The current study suggested that the changes in CA9 gene expression level might be used as a predictive marker to assess early prognosis at the time of detection of de novo leukemia, and then monitor tumor progress during treatment. This study highlights the level of CA9 gene expression in leukemic patients. A total of 44 cases (acute myeloid leukaemia (AML) group: 23 cases; acute lymphoid leukaemia (ALL) group: 13 cases; control group: 8 healthy volunteers) were selected for this study. The CA9 gene expression was assessed by a real-time PCR with the SYBR green assay. A high level of CA9 gene expression was noticed in AML patients compared to the control group, while the results were not significant in ALL patients. After treatment follow-up, significant differences were observed in CA9 gene expression between a complete response and no response in AML patients. As a result, the CA9 tumor gene could act as a potential early marker for acute leukemia prognosis. A low level of CA9 expression was associated with better clinical outcomes, while a high level was related to a negative prognosis in patients with AML.