Although definitions of mass gatherings (MG) vary greatly, they consist of large numbers of people attending an event at a specific site for a finite time. Examples of MGs include World Youth Day, the summer and winter Olympics, rock concerts, and political rallies. Some of the largest MGs are spiritual in nature. Among all MGs, the public health issues, associated with the Hajj (an annual pilgrimage to Mecca, Saudi Arabia) is clearly the best reported-probably because of its international or even intercontinental implications in terms of the spread of infectious disease. Hajj routinely attracts 2·5 million Muslims for worship. WHO's global health initiatives have converged with Saudi Arabia's efforts to ensure the wellbeing of pilgrims, contain infectious diseases, and reinforce global health security through the management of the Hajj. Both initiatives emphasise the importance of MG health policies guided by sound evidence and based on experience and the timeliness of calls for a new academic science-based specialty of MG medicine.