Changes in T2 asymmetry in subjects with acute low back pain (LBP) following a single osteopathic manipulative treatment (OMT) session. Data were obtained before manipulation (baseline), immediately following OMT and 48-hours following OMT. T2 asymmetry was calculated as the absolute value of the percent difference in MRI-derived T2 between the left and right side muscles. Immediately following OMT the T2 asymmetry in the psoas muscle was reduced, but it returned to baseline levels after 48-hours although a modest effect size was still observed (+p = 0.06, eta2 = 0.41). Conversely, 48-hours following OMT a small, but significant, increase in multifidus T2 asymmetry was observed (*p = 0.01, eta2 = 0.67). The quadratus lumborum and iliocostalis/longissimus muscles exhibited modest effect sizes for reduced T2 asymmetry associated with OMT (+ eta2 = 0.19 and 0.17, respectively), although these differences failed to reach significance (p = 0.23 and 0.27, respectively).