BACKGROUND:As a member of the Rho small guanosine triphosphatase family, ras-related C3 botulinum toxin substrate 1 (RAC1) interacts with various specific effectors, and p21-activated kinase 1 (PAK1), which has a role in both carcinogenesis and cellular invasion, binds to RAC1, after which activated PAK1 regulates cellular functions. There have been few reports about the simultaneous analysis of RAC1 and its downstream effector PAK1 in upper urinary tract urothelial carcinoma (UTUC). We assessed the expressions of both RAC1 and PAK1 and evaluated their association with clinicopathological parameters. METHODS:Immunohistochemical studies of RAC1 or PAK1 were performed with specimens from 104 patients with N0M0 UTUC and cancer-free surgical margins. Correlation of the positive expression of RAC1 or PAK1 or both with clinicopathological parameters was evaluated. RESULTS:A hazard model showed that the presence of mixed histologic features and moderate or strong positive expression of both RAC1 and PAK1 were independent factors for shortened disease-specific survival time (Ps = 0.041 and 0.016, respectively), and another hazard model revealed that only moderate or strong positive expression of both RAC1 and PAK1 was an independent factor for shortened recurrence-free survival time in the multivariate analysis (P = 0.036). Neither moderate or strong positive expression of RAC1 alone nor moderate or strong positive expression of PAK1 alone was an independent factor for a worse rate of disease-specific or recurrence-free survival in multivariate analysis. CONCLUSIONS:Patients with N0M0 UTUC, cancer-free surgical margins and moderate or strong positive expression of both RAC1 and PAK1 should be carefully monitored after surgery.