We previously reported that cisplatin induced skeletal muscle atrophy in mice. Myosin heavy chain (MyHC) is one of the structural proteins essential for the contractile response of skeletal muscle, and several isoforms exist. However, the kinetics of its expression in cisplatin-induced muscle atrophy is unknown. In this study, we investigated the expression changes of MyHC isoforms in cisplatin-induced muscle atrophy. Mice were treated with cisplatin intraperitoneally once daily for 4 days, and quadriceps muscles were isolated 1 day after the last dose; C2C12 myotubes were also treated with cisplatin. Cisplatin significantly reduced protein expression of MyHC isoforms (I, IIa, IIx, IIb) in mice compared to controls. However, only MyHC-IIa was downregulated when changes in MyHC gene expression were examined. Similar to the results in mice, cisplatin attenuated protein expression of all MyHC isoforms in C2C12 myotubes, and gene expression of MyHC-IIa was downregulated. Furthermore, the downregulation of protein expression of all MyHC isoforms was inhibited by treatment with MG-132, a proteasome inhibitor. These results indicate that cisplatin downregulates MyHC protein expression. Thus, we suggest that proteasomal degradation of MyHC proteins is one of the causes of cisplatin-induced muscle atrophy.