World Youth Day, a youth-focused Catholic celebration, is the largest recurring youth event in the world. The 10th International World Youth Day (WYD) was held from 15-20 July 2008, in Sydney, Australia and attenders participated in a week-long series of religious events and activities. Little research in English has been conducted on WYD generally, and specifically on the impact participation has on the faith of attenders. This article investigates the impact participation at this event had on Mass attendance post-WYD. Data are from a representative on-line survey conducted five months after the event among registered WYD English-speaking attenders (n = 2483). The results of multivariate analysis reveal change is most likely among those with moderately high levels of pre-WYD religious practice, rather than those with the lowest or highest pre-WYD levels of religious practice. In addition, those who enjoy the community solidarity and religious experiences at the event are more likely to report a post-WYD increase in Mass attendance. (Contains 2 tables and 3 notes.)