Background: Spondylolysis is a defect of the pars interarticularis and is most commonly encountered in the lower lumbar spine of pediatric patients. Stress reaction of the pars interarticularis was regarded as an early stage of this pathological diagnosis in which no obvious anatomical change is detected. Both SPECT with Tc-99m MDP and CT of the lumbar spine have been used to evaluate the patients with low back pain. The purpose of this study was to determine the prevalence of stress reaction in pars interarticularis in pediatric patients presenting new-onset back pain using both bone SPECT and CT. Patients and Methods: The records and images of 63 consecutive pediatric patients (mean age, 13.2 years) with new onset of back pain, who received both Tc-99m MDP bone scintigraphy with SPECT of the lumbar spine and lumbar thin-slice CT were retrospectively analyzed. The results of bone SPECT and CT were retrospectively compared. Results: Among the cohort of 63 patients who had both bone SPECT and thin-slice CT of the lumbar spine, there was a total of 56 positive SPECT results indicating stress injuries in the pars interarticularis. Spondylolysis was shown on both bone SPECT and thin-slice CT in 45 patients. In 11 patients, stress reaction was shown. In 7 patients, no abnormality was detected by either SPECT or thin-slice CT. Conclusions: Our result showed that a significant portion (19.6%, 11 of 56) of the stress injuries in the pars interarticularis will be CT-negative stress reaction in pediatric patients with new-onset back pain.