Introduction : Benign acute childhood myositis is a self-limiting process characterized by sudden onset of muscle pain. Patients often refuse to walk or have difficulty walking due to pain (more often calf pain), or true muscle weakness. Benign acute childhood myositis should be included in the differential diagnosis of children with sudden difficulty walking. We report a 4-year-old patients with benign acute childhood myositis. Case : A 4-year-old boy who had medication for a month due to fever, cough, coryza, was admitted complaining of difficulty walking suddenly from one day before visit our hospital. He had no trauma history. One day before admission, he suddenly refused to walk, and cried when his parents touched his legs after nap. On physical examination, his throat was injected, tonsil was hyperemic, and breathing sound was clear. He had not swelling, skin change, bony abnormality on legs, but had direct tenderness on both calf and thigh. Neurologic examination showed normal muscle power and tone. Laboratory findings showed Hb 12.5g/dL; hematocrit (Hct) 37.3%; WBC count 4,090 μL; platelets count 108,000/μL; AST(GOT) 96IU/L; ALT(GPT) 26IU/L; CRP 0.43mg/L; CK 1555IU/L; Urinalysis and viral study showed nonspecific findings. We decided to give him dexibuprofen, and observe him. On second hospital day, he started walking, and had little pain, and on third day, he was running, and had no pain at all. At follow up laboratory findings at third day, CK level was 861IU/L, so he was discharged that day. After five days, we performed last follow up laboratory test at outpatient department, and CK level was 215IU/L. Conclusion: A 4-year-old boy complained of difficulty walking and leg pain after upper respiratory tract infection. He had CK elevation level at laboratory test and tenderness on both calf and thigh. We considered more severe and complex disease such as Guillain-Barre syndrome, hypokalemic periodic paralysis. But neurologic examination was in normal range, and he showed symptom improvement after taking dexibuprofen, we could diagnosis benign acute childhood myositis. When children have sudden difficulty walking, it is important to make a correct differential diagnosis including from benign acute childhood myositis to Guillain-Barre syndrome or rhabdomyolysis, etc.