This paper presents findings on potential frequency-response-related impacts of increased variable renewable wind generation on North America's western interconnection by 2012. The assessment was conducted using commercially available, production-grade dynamic simulation tools and industry-developed system models, which include the amount of wind generation capacity that planners in the interconnection expect in 2012. Considering the largest generator outage routinely studied, for the high operating reserve case, the frequency nadir remains well above the highest under-frequency load-shedding set point, which is 59.5 Hertz (Hz). The initial rates of frequency decline are nearly identical, but the frequency nadirs are slightly lower as the level of wind generation increases. For the low operating reserves cases, the frequency nadirs are close to, but still above, the highest under-frequency load-shedding set point. The initial rates of frequency decline are slightly greater and the frequency nadirs are noticeably lower as the level of wind generation increases.