Rhabdomyosarcoma (RMS) is a common soft-tissue sarcoma of childhood in need of more effective therapeutic options. The expression of p53 in RMS is heterogeneous such that some tumors are wild-type whereas others are p53 mutant. The small molecule CP-31398 modulates both the wild-type and the mutant p53 proteins. Here, we show that CP-31398 blocks the growth of RMS cells that have either wild-type or mutant p53 status. In wild-type A204 cells, CP-31398 increased the expression of p53 and its downstream transcriptional targets, p21 and mdm2; enhanced the expression of apoptosis-related proteins; and reduced proliferation biomarkers. Flow profiling of CP-31398-treated cells indicated an enhancement in sub-G0 and G1 populations. CP-31398 inhibited proliferation in a manner associated with co-induction of SOX9 and p21. Apoptosis induced by CP-31398 occurred with translocation of p53 to mitochondria, leading to altered mitochondrial membrane potential, cytochrome c release, and reactive oxygen species release. In vivo, CP-31398 decreased the growth of tumor xenografts composed of wild-type or mutant p53 tumor cells, increasing tumor-free host survival. Our findings indicate that the ability of CP-31398 to modulate wild-type and mutant p53 results in the inhibition of RMS growth and invasiveness. ©2010 AACR.