Problem, research strategy, and findings: Mega-events such as the Olympic Games and the Football World Cup have become complex and transformative undertakings over the last 30 years, with costs often exceeding USD $10 billion. These events are currently planned and governed in ways that produce adverse effects for cities, regions, and residents. This study identifies a mega-event syndrome, a group of symptoms that occur together and afflict mega-event planning, including overpromising benefits, underestimating costs, rewriting urban planning priorities to fit the event, using public resources for private interest, and suspending the regular rule of law. I describe each of these symptoms, providing empirical examples from different countries and mega-events, examining the underlying causes. The research is based on material from field visits to mega-event sites in 11 countries as well as 51 interviews with planners, managers, politicians, and consultants involved in mega-event planning.Takeaway for practice: To curb the mega-event syndrome, I propose both radical and incremental policy suggestions. The most crucial radical change that an event host could make is to not tie mega-events to large-scale urban development, avoiding higher risks that create cost overruns, substandard construction quality, and oversized infrastructure not suitable for post-event demands. Further, event hosts should bargain with event-governing bodies for better conditions, earmark and cap public sector contributions, and seek independent advice on the costs and benefits of mega-events. Event-governing bodies, for their part, should reduce the size and requirements of the events.