The UK's busiest airport, Heathrow, saw 70 million passengers pass through its terminals in a 12-month period for the first time in March.
BAA, which also owns Stansted, Edinburgh, Glasgow, Aberdeen and Southampton Airports, said the "major milestone" reflected the airport's "resilience" amid challenging economic conditions.
Heathrow, which has five terminals, dealt with 5.7 million passengers in March, a 6.9% rise on the previous year as pre-Easter traffic occurred in March, rather than April.
BAA airports saw a 4% rise in passengers in the month to 8.6 million as growth at Glasgow and Aberdeen was offset by declines at Stansted, Edinburgh and Southampton.
The airline operator, part owned by Spain's Ferrovial, is working on another legal challenge against a requirement to sell Stansted, after it lost an appeal against the decision earlier this year.
Colin Matthews, BAA chief executive, said: "Reaching 70 million passengers at Heathrow is a major milestone, demonstrating the resilience of the airport in an otherwise challenging economic environment."
Heathrow saw an improvement in its load factor, a measure of the use of available capacity on aircraft at the transport hub, which rose by 4.2 percentage points to 73.4% compared with March last year.
Elsewhere, Glasgow recorded a 4.6% increase in traffic, while Aberdeen saw a 10.2% rise.
Stansted, Edinburgh and Southampton were down 4.7%, 2.8% and 1.7% respectively, with a drop in domestic traffic driving the decline at Edinburgh and Southampton.
North Atlantic traffic was up 13.6% across the group, BAA said, while other long haul increased 5.2%. European scheduled traffic, the group's biggest market, was ahead 3.2%.
The strong ski season boosted chartered flights to Europe, BAA added.
BAA gave no update on its latest appeal against the Competition Commission's ruling that it must sell Stansted in Essex.
BAA has mounted a series of legal challenges to the decision, in which time it has also sold Gatwick Airport in West Sussex and, faced with having to dispose of either Edinburgh Airport or Glasgow Airport, opted to sell Edinburgh.