The purpose of this study was to report the results of a specific treatment protocol for athletes with spondylolysis or spondylolisthesis of the lumbar spine. A retrospective study with recent follow-up was performed on 82 patients treated with restriction of activity, bracing, and physical therapy. All of the patients were involved in sports at first onset of symptoms. Sixty-six patients were boys and 16 were girls. Activities involving repetitive hyperextension and/or extension rotation of the lumbar spine were described as painful in 98% of the patients. Of the 62 patients with spondylolysis, 53 (85%) had an L5 defect and nine (15%) an L4 defect (90% of these 62 patients' defects were located in the most caudad mobile vertebra). Thirty-seven patients had bilateral pars defects, and 25 had unilateral defects. Eight patients had normal roentgenograms, but these eight had abnormal bone scans. Nine patients with spondylolysis underwent posterolateral fusion. Average follow-up was 4.2 years. Fifty-two (84%) had excellent results, eight had good results, and two had fair results. Twenty patients had a spondylolisthesis: 12 were grade I, six were grade II, and two were grade III. Twelve patients (60%) required surgery; 9 had excellent results, one had good results, one had a fair result, and one had a poor result. Pars defects must be suspected in the differential of low back pain in young athletes. Oblique radiographs are frequently diagnostic; however, if the history and examination are suggestive despite normal plain films, a bone scan should be obtained. Nonoperative management of pars defects is frequently successful.(ABSTRACT TRUNCATED AT 250 WORDS)