Hemoglobin F (HbF) concentration is used in the diagnosis of certain hemoglobinopathies and accurate quantification is central to treatment of patients with sickle cell disease. The 2 most commonly used methods to quantify HbF are high performance liquid chromatography and capillary zone electrophoresis. This study reports discrepancies in HbF quantification between these methods when hemoglobin S is present in the sample. Clinicians and investigators should be mindful of the method used for HbF quantification when evaluating and treating patients who produce hemoglobin S.