We retrospectively identified all children with acute hip pain who underwent pelvic magnetic resonance (MR). Children with septic hip or history of trauma were excluded; the remaining children with signs of infection (fever, >38°C; leukocytosis, >12 × 109/L; or elevated erythrocyte sedimentation rate [ESR], >30 mm/h) comprised the study group. Thirty-three children (9 girls; age, 0.8-15.8 years) were identified. On MR examination, 18 (55%) of 33 children had hip joint effusion, whereas 19 (58%) of 33 children had other abnormalities, including pyomyositis (n = 15), osteomyelitis (n = 12), and sacroiliitis (n = 3). Staphylococcus aureus was cultured from 13 (68%) of these 19 children. Compared with MR, sensitivity for bone and soft tissue abnormalities was 30% for pelvic radiography (n = 26) and 71% for bone scintigraphy (n = 8). Elevated ESR (>30 mm/h) was the clinical finding that best predicted pelvic osteomyelitis or pyomyositis. Pelvic MR should be performed to rule out pelvic osteomyelitis or pyomyositis in children with acute hip pain, ESR of more than 30 mm/h, and no evidence of septic hip.