In preparing for medical coverage of a mass participation event such as a marathon, race directors and their medical staff members need to account for the unexpected. Extremes in weather as well as the potential for outside threats need to be given consideration before race day in order to adequately prepare. Through the recruitment of local expertise from various agencies in one's community during both the planning stages, and on race day, the added stressors of such extremes will be minimized, if not eliminated. This article will provide concrete examples of how the Chicago Marathon has used its own experiences with such extremes. Readers will be given useful tools to implement in their own marathons or other mass participation events-planning to equip them better for the unexpected surge.