Avulsive cortical irregularity, a benign condition occurring only among children and adolescents, has been known to simulate malignancy not only radiologically but also microscopically. Therefore, in addition to plain radiographs, further studies including by magnetic resonance (MR) imaging may occasionally be required. MR images of seven cases of avulsive cortical irregularity of the femur were reviewed. In all cases, the lesion appeared hypointense on T1-weighted images and hyperintense on T2-weighted images, with a dark rim on both sequences at or near the sites of the bony attachment of the medial head of the gastrocnemius muscle. In all cases, bilateral involvement was demonstrated by plain radiography, computed tomography, and/or MR imaging. The authors suggest that avulsive cortical irregularity involves both femora much more frequently than has been reported previously.