Sixty-seven persons with symptomatic spondylolysis or grade 1 spondylolisthesis were treated with the modified Boston brace. The average age was 16.0 years, and the average follow-up was 2.5 years. Following treatment, 52 persons (78%) had either an excellent or good result with no pain and returned to full activities. Nine (13%) continued to have mild symptoms, and six (9%) subsequently required fusion in situ. Twelve of the patients showed radiographic evidence of healing of their pars defect(s). This group and those with the best overall results tended to be men with spondylolysis and relatively acute onset of symptoms. Age, delay in treatment, spina bifida, and bone scan result did not correlate with the ultimate clinical result.