Middle East respiratory syndrome coronavirus (MERS-CoV) has emerged in the Arabian Gulf region, with its epicentre in Saudi Arabia, the host of the 'Hajj' which is the world's the largest mass gathering. Transmission of MERS-CoV at such an event could lead to its rapid worldwide dissemination. Therefore, we studied the frequency of viruses causing influenza-like illnesses (ILI) among participants in a randomised controlled trial at the Hajj 2013. We recruited 1038 pilgrims from Saudi Arabia, Australia and Qatar during the first day of Hajj and followed them closely for four days. A nasal swab was collected from each pilgrim who developed ILI. Respiratory viruses were detected using multiplex RT-PCR. ILI occurred in 112/1038 (11%) pilgrims. Their mean age was 35 years, 49 (44%) were male and 35 (31%) had received the influenza vaccine pre-Hajj. Forty two (38%) pilgrims had laboratory-confirmed viral infections; 28 (25%) rhinovirus, 5 (4%) influenza A, 2 (2%) adenovirus, 2 (2%) human coronavirus OC43/229E, 2 (2%) parainfluenza virus 3, 1 (1%) parainfluenza virus 1, and 2 (2%) dual infections. No MERS-CoV was detected in any sample. Rhinovirus was the commonest cause of ILI among Hajj pilgrims in 2013. Infection control and appropriate vaccination are necessary to prevent transmission of respiratory viruses at Hajj and other mass gatherings.