This is the first case report of a child with isthmic spondylolisthesis and discitis who had spontaneous fusion develop at an unstable level with relief of symptoms after nonoperative treatment. Although the blood culture was negative, the 14-year-old boy with Grade III isthmic spondylolisthesis of L5 was diagnosed with discitis at the L5-S1 level, based on clinical findings, elevated C-reactive protein, plain radiographs, and magnetic resonance imaging scans. The patient was treated with antibiotics for 19 weeks and bed rest for 4 weeks followed by immobilization in a hip spica cast for 8 weeks and a thoracolumbosacral orthosis for an additional 12 weeks. The lumbar back pain improved and there was a decrease in C-reactive protein to the normal range 3 weeks after onset. Forty months from onset, the patient was free from lumbar back or leg pain and his clinical neurologic examination was normal. Plain radiographs showed spontaneous fusion between L5 and the sacrum. This suggests that nonoperative treatment is acceptable even if discitis occurs at an unstable level.