To evaluate the natural history of spondylolysis and spondylolisthesis, for clinical and radiological prognostic factors for progression of slip and the need for operative intervention, 47 patients with symptomatic spondylolysis or low-grade isthmic spondylolisthesis were studied retrospectively. The mean follow-up time was 7 years. Two patients (4%) had a progression of slip. Thirty percent of the patients required operation after a mean observation period of 3.7 years. At follow-up, 83% of the nonoperatively treated patients were rated excellent or good. No prognostic factor for progression of slip or need for future operative treatment was found.