Introduction: Approximately 45% of adults with acute myeloid leukemia (AML) have normal karyotypes and therefore lack structural abnormalities that can assist in the localization and characterization of molecular defects. The current study attempted to evaluate the potential prognostic role of multidrug resistance (MDR), regarded as one of the potential prognostic factors for the outcome of overall AML, for AML with normal karyotypes. Method and Materials: A functional MDR assay was performed in pretreatment samples from AML patients with normal karyotypes. The complete remission (CR) rate, event-free survival (EFS), and overall survival (OS) were analyzed according to the MDR status and clinical prognostic factors for 88 patients with AML with normal karyotypes. Results: MDR by efflux was expressed in 14 out of 48 evaluable patients (29%) but failed to identify the association with CR (p = 0.124). However, MDR was identified as an independent prognostic factor for EFS and OS (p = 0.013 and 0.046) together with the use of stem cell transplantation (p = 0.009 for EFS and 0.029 for OS) and the WBC count at presentation (p = 0.023 for EFS and 0.034 for OS). Conclusion: The functional MDR assay may provide information on the prognosis of AML patients with normal karyotypes, and it might be a possible guideline for risk-stratified treatment strategies in AML with normal karyotypes.