International Airline Group saw traffic grow by 4% last month over April 2014.
This came as the British Airways, Iberia and Vueling parent company increased capacity by 6.4%.
Group premium traffic for the month increased by 5.3% compared to the previous year.
Overall passenger carryings rose by 7.9% to more than 6.8 million, with the annual rolling total up by 8.7% to 23.5 million.
IAG has taken delivery of five Airbus A320s which have joined Vueling’s fleet.
The aircraft are part of a harmonisation plan aimed at reducing costs by standardising the A320 fleet across the group.