Stress fracture of the pars interarticularis is an increasing cause of disability in highly competitive adolescent athletes. We have documented this lesion in 14 adolescent athletes engaged in repetitive training and competition exercises involving flexion/extension of the lumbar spine. An in vivo spinal muscle torque study in 11 normal adolescent girls also revealed mean torque values of 22.6 Nm for left lateral flexion and 27.4 Nm for hyperextension. The most common sports resulting in this lesion were gymnastics and hockey. In four patients the lesion was bilateral, and in 10 it was unilateral. Five of the unilateral lesions went on to heal with immobilization in a thoracolumbar spinal orthoses; however, none of the four bilateral lesions or the remaining five unilateral lesions healed in spite of 3 months of immobilization.