Background: Children with septic arthritis and concomitant osteomyelitis have been reported to have significantly worse outcomes than those with septic arthritis alone. However, there is no evidenced-based consensus on the risk factors for concomitant osteomyelitis and septic arthritis. The purpose of this study was to investigate clinical parameters predictive of concomitant osteomyelitis in children with septic arthritis. Methods: Retrospective review was conducted on all septic arthritis patients between January 2004 and October 2016 at a tertiary care pediatric hospital. Medical charts were reviewed for information including symptoms, diagnosis of osteomyelitis, serum laboratory studies, joint fluid analyses, imaging results, and treatment. Positive diagnosis of osteomyelitis was defined as hyper-intense signaling of osseous structures on T2-weighted magnetic resonance imaging (MRI) consistent with infection per attending pediatric radiologist final read. Results: 71 patients with 73 septic joints were included. Mean age was 6.9 ± 4.6 (0.1-17.7) years and mean follow-up was 14.9 ± 24.1 (1.0-133.1) months. Septic arthritis with concomitant osteomyelitis occurred in 43/71 (61%) patients while 28/71 (39%) patients had septic arthritis alone. Inflammatory markers such as white blood cell count, erythrocyte sedimentation rate, and C-reactive protein on admission were not associated with concomitant osteomyelitis. Multivariate logistic regression revealed that positive joint fluid bacterial culture (p= 0.011) and pain for > 4 days prior to admission (p = 0.001) are independent risk factors for concomitant osteomyelitis in children with septic arthritis. Among the 24 septic arthritis patients with pain for > 4 days prior to presentation, 96% (23/24) had concomitant osteomyelitis while 43% (20/47) of patients with pain for ≤ 4 days had concomitant osteomyelitis. Conclusion: Pain for > 4 days prior to presentation is an independent predictor of osteomyelitis in children with septic arthritis. In pediatric septic arthritis patients presenting with pain for > 4 days, obtaining an MRI should strongly be considered to evaluate for osteomyelitis as 96% of these patients had concomitant osteomyelitis.