Herein, we report on the correct diagnosis and effective treatment procedures for pyomyositis, a very rare complication that remains a diagnostic challenge in children being treated for acute lymphoblastic leukemia (ALL). We report the case of a 10-year-old girl suffering from pyomyositis with ALL. Correct diagnosis is usually delayed because the initial symptom of pyomyositis, usually local pain, is similar to the common side effect of vincristine, a drug necessary for ALL induction therapy. We summarize the procedures taken to reach a timely diagnosis and therapeutic success.