This paper presents a method for measuring 3D left ventricular (LV) strain from phase unwrapped harmonic phase (HARP) images derived from tagged cardiac magnetic resonance imaging (MRI). Limited user interaction is needed, but, in contrast to existing techniques, 3D strains can be measured over the entire LV. In addition, unwrapped phase is more robust to interframe motion since it only requires that the average interframe deformation be less than one-half tag spacing. The unwrapped-phase-derived strains were validated on a set of 30 human studies by comparing them to strains estimated by a feature-based technique. The standard deviation of the difference between strains measured by the two methods was less than 5% of the average of the strains from the two methods.