BACKGROUND: We noticed that most of active sports children with low back pain (LBP) have muscle tightness around the pelvis and reduced trunk flexibility. Abnormalities in short-time inversion recovery (STIR) images on magnetic resonance imaging (MRI) can show stress fracture. Therefore, we investigated the associations among LBP, trunk flexibility, and lumbar stress fractures. METHODS: A total of 130 patients under the age of 18 years complained of LBP were investigated in STIR MRI images. Among these 130 patients, 65 cases of lumbar stress fracture were diagnosed and 65 cases were not diagnosed as a lumbar stress fracture. We compared between a group suspected of stress fracture (suspected group) and a group of stress fracture(stress fracture group)about their trunk flexibility. These groups were investigated about their initial trunk flexibility about below items; Finger floor distance (FFD), Heel to buttock distance (HBD), straight leg raising (SLR). RESULTS: Significant differences were observed between suspected group and stress fracture group about every items; SLR (P < 0.001), FFD (P < 0.01), HBD (P < 0.002). Most cases of stress fracture group had reduced trunk flexibility, and low flexibility in pelvic area muscles was observed in 93.8% (61/65) of cases at the initial examination. Otherwise, that of suspected group was 73.8%(48/65). CONCLUSIONS: Most patients of lumbar stress fracture had reduced trunk flexibility, and their reduced trunk flexibility might not be caused by LBP. In the early diagnosis of lumbar stress fractures using STIR MRI images, there were indicated that reduced trunk flexibility was one of helpful item for lumbar stress fracture.