Background: Studies on the serum concentration of hyaluronic acid (HA) in newly diagnosed patients with acute myeloid leukemia (AML), B-acute lymphoblastic leukemia (B-ALL), and mantle-cell lymphoma (MCL) are scarce. In this study, we focused on investigating whether HA could serve as a possible prognostic marker in patients with AML, B-ALL, and MCL. Methods: The serum concentration of HA was measured in a total of 51 patients with newly diagnosed AML, B-ALL, and MCL. Venous blood was collected 1 day before the initiation of chemotherapy (D0), on day 16 of the first cycle of chemotherapy (D16), and on D30. Results: The serum HA concentration on D0 in patients with AML, B-ALL, and MCL was higher than in the control group. For all types of hematological malignancy, on D0, serum HA values of nonsurvivors were higher than in survivors. Moreover, patients in relapse had higher levels of serum HA than patients in remission. A strong positive correlation between serum HA and ferritin, β2-microglobulin, and lactate dehydrogenase was found. Conclusion: Serum HA may serve as a possible prognostic marker for AML, B-ALL, and MCL patients, especially on D0. Prospective case-control studies on larger populations may provide further information.