Lumbar foramimal stenosis is one of the important disorders underlying radicular symptoms and unrecognized or recurrent foraminal stenosis may result in failed back surgery. We report a 47-year old man with foraminal stenosis of the lumbar spine associated with a bone spur at the pars interarticularis. He suddenly experienced severe right leg pain, lumbago and gait disturbance. Radiographic studies revealed foraminal stenosis on the right L5/S1associated with a bone spur at the pars interarticularis. There was no dural sac compression by spondylolysis. As his symptoms failed to respond to conservative treatment, he underwent decompression without fusion with an ultrasonic bone curette. We performed lateral fenestration and foraminotomy at the right L5/S1 using the extraforaminal approach. After partial foraminotomy, the right L5 nerve root was compressed by the bone spur at the pars interarticularis. For satisfactory decompression we removed the bone spur using an ultrasonic bone curette. His symptoms disappeared immediately after surgery. Although long-term follow-up is necessary, we suggest that microdecompression with an ultrasonic bone curette is a useful method to treat patients with foraminal stenosis of the pars interarticularis.