There is a need to evaluate the effectiveness of stuttering treatment programs delivered in domestic and international contexts and to determine if treatment delivered internationally is culturally sensitive. Evaluation of the effectiveness of the ISTAR Comprehensive Stuttering Program (CSP) within and across client groups from the Netherlands and Canada revealed generally positive results. At 2 years post-treatment both groups were maintaining statistically significant reductions in stuttering frequency and improvements in attitudes, confidence, and perceptions as measured by the Revised Communication Attitude Inventory (S24), Perceptions of Stuttering Inventory (PSI), and the approach scale of the Self-Efficacy Scaling by Adult Stutterers (SESAS). Data pooled across the groups on these measures gave evidence of a global treatment effect with standardized effect sizes ranging from typical to larger than typical in the behavioural sciences. Only two differences between the groups emerged: differences in speech rate and perception of self. Given that these groups represent two distinct cultures, differences were discussed in terms of whether they could be due to cultural, methodological, or other variables. Overall, results suggest that, the CSP appears to be similarly effective in both cultures and thus, sufficiently sensitive to the culture of Dutch adults who stutter. Educational objectives: The reader will be able to (a) describe a methodology that can be used in a clinical setting to evaluate the long-term effectiveness of stuttering treatment with adults, (b) describe some of the challenges in developing a model of clinically meaningful outcome, (c) explain the rationale for the need for cross-cultural investigations of treatment outcome, and (d) summarize speech and self-report results of the cross-cultural evaluation of an integrated stuttering treatment program. (Contains 4 tables and 1 figure.)