Muscle side effects have not been frequently assessed in childhood acute lymphoblastic leukemia (ALL) patients. Our objective was to determine the early and late muscle side effects during childhood ALL treatment. To this end, we examined the early muscle side effects in 15 newly diagnosed "therapy patients" (group I), and the late side effects in 18 ALL survivors "off therapy patients" (group II). Muscle side effects were assessed by measuring hand grip strength (HGS), the "timed up and go" (TUG) test, creatine phosphokinase, myoglobin, plasma electrolytes, cardiac troponin I before and after induction chemotherapy in group I. The same parameters and cranial radiotherapy dose were examined in group II at a single timepoint. Cumulative doses of chemotherapy were calculated, and echocardiograms were obtained for each group. As a result, we found that the HGS and TUG measures of group I at the end of induction were poorer than measures of the first admission, control subjects, and group II. Low HGS measures had been normalized over time, but the TUG measures remained poor. Overt rhabdomyolysis and pyomyositis were not detected in any of the patients. These results suggested that muscle side effect monitoring might be helpful in the follow-up of children with ALL.