Study Design.An operator-blinded dual modality trial of measurement of lumbar multifidus muscle cross-sectional area was performed.Objective.To compare two imaging modalities used for measurement of the lumbar multifidus muscle.Methods.Ten normal female subjects aged 21–31 years were imaged on two separate days using magnetic resonance imaging and ultrasound imaging. Bilateral measurements were made at each vertebral level from L2-S1.Results.No significant differnce was demostrated between cross-sectional area measurements made with the two different modalities. measurements of multifidus were symmetrical between left and right sides of the spine. For both modalities, a significant difference was demostrated in the cross-sectional area of multifidus between each vertebral level from L2-S1.Conclusions.The present study indicated that if a strict protocol for ultrasound imaging is adhered to, real-time ultrasound imaging can be used to document muscle size in young adults. Further studies are required to validate the technique in older subjects and in different conditions.