After two decades of lying nearly dormant, Dulles International Airport emerged last year as the nation's fastest growing airport.

Statistics released yesterday by the Washington Dulles Task Force, a private organization that promotes the growth of the airport, show that more than 5 million people passed through the sloping glass terminal at Dulles in 1985.

The Federal Aviation Administration said the rapid growth, spurred mostly by the decisions of five airlines to establish hubs at Dulles, represents a jump in travelers of nearly 50 percent over the previous year.

"I never imagined it was possible to move this fast," said Thomas G. Morr, president of the task force. "We are talking about 150 new departures every day in the past year. And many of the airlines that are here now have plans to expand."

Dulles was built in 1962 as the country's first jetport, but has always played second fiddle to National Airport, one of the most heavily used airports in the nation. Recently, as deregulation has encouraged the growth of hubs, and demographic patterns have shifted in the Washington region, Dulles has come alive.

Pan American Airlines, New York Air and Presidential Airways began hub service -- major centers for departures and arrivals -- during 1985. In addition, United Airlines announced plans in December to create a major hub. The company is building a $15 million, 10 to 12 gate facility to accommodate its service, which is to begin in May.

And, in connection with United's arrival, Air Wisconsin announced it will begin flying to Dulles this spring.

Dulles' biggest potential problem now, as always, is growth. But now the concern is that it may expand faster than facilities will allow. Local government officials fear that unless Congress approves a transfer of Dulles and National from the federal government to a local authority or awards Dulles the money it needs to expand, the growth at the airport could stop in the next two years.