The purpose of this study was to clarify the efficacy of conservative treatment in athletes with severe low back pain and spondylolysis, especially focusing on returning to original sporting activities. One hundred and four athletes (96 males and eight females), who consulted our sports medicine clinic during the 11-year period between September 1991 and October 2002 because of low back pain with an apparent defect of the pars interarticularis on plain radiographs, were subjected to conservative treatment. The mean age of the patients was 20.7 years. Of all the patients, 40 (38.5%) discontinued sporting activities due to severe low back pain, and were conservatively treated with activity restriction and antilordotic lumbosacral bracing, aimed at relieving low back pain by achieving stability of the fracture partly associated with fibrous union. After their low back pain was markedly reduced, the brace was removed and then individual training to return to the original sporting activities was started. Thirty-five patients (87.5%) could return to their original sporting activities in an average of 5.4 months (range: 1.0–11.5 months) after the onset of treatment, and could continue the activities for at least 6 months despite non-bony union. These results suggest that the outcome of conservative treatment with activity restriction and bracing appears to be satisfactory in controlling symptoms and returning to original sporting activities. Randomized controlled trials or comparative follow-up studies are needed to confirm our results.