To evaluate fat infiltration in the multifidus muscle close to the scoliotic curve as seen in MRI compared with the histological evaluation in patients with adolescent idiopathic scoliosis (AIS). The evaluation of fatty infiltration in the multifidus muscle of patients with AIS by MRI was compared with biopsy. Fatty infiltration was greater in concave side than in the convex side in both MRI (P=0.005) and biopsy evaluation (P=0.026). There was no correlation between the fatty infiltration findings evaluated through MRI and biopsy. The MRI measurements do not express accurately the tissue changes observed in the biopsy.