The lumbar spine was assessed by magnetic resonance imaging (MRI) in 14 children (mean age 12.4 years) with unspecific activity-related low back pain for more than 3 weeks presenting with normal plain radiographs. Impending spondylolysis was diagnosed when typical signal abnormalities were confined to the pars interarticularis without signs of thinning or fragmentation. After brace treatment for 3 months, follow-up MRI was performed 3 and 6 months after treatment. MRI signals returned to normal after 3 months in six patients and after 6 months in one patient. MRI showed promising results in detecting and monitoring the early onset of spondylolysis. Bracing and avoiding strenuous activities prevented the formation of pars defects in all our patients.