The distal femur is a common site for benign or malignant bone tumors in children or adolescents. Distal femoral cortical irregularities at the posterior aspect of the distal femoral metaphysis must be differentiated from malignant bone tumors because they might be misinterpreted as malignant neoplasm. Plain radiographs of a 6-year-old girl complaining of left knee pain for 4 weeks showed cortical proliferation with excavation on her distal femoral metaphysis. Computed tomography, magnetic resonance imaging and bone scan helped to differentiate the cortical irregularity from malignant lesions. Therefore unnecessary invasive surgery was avoided in this patient.