An Arlington circuit court judge yesterday sentenced a 25-year-old Vietnam veteran, who police believe committed at least 725 burglaries in the Washington area in the last three years, to 20 years in prison.

William Lee Kidd, known to police as "Billy the Kidd," was prosecuted on 15 counts of breaking and entering and was sentenced yesterday to 40 years in prison with 20 years suspended after pleading guilty to the crimes.

Arlington police Sgt. John Karinshak said yesterday that those counts were just the tip of Kidd's criminal activity.

Karinshak said Kidd's statements, crime-scene clues, evidence found on him at the time of his May 13 arrest and his knowledge of crime details available only to the criminal have led thus far to the closing of hundreds of burglary cases in Arlington, Falls Church, Alexandria and the District of Columbia.

"When we first brought him in and he started confessing to all these burglaries, I thought he was feeding us a line of bull," Karinshak said. "But they checked out . . . We've given him a lie detector test and he passed. . . We feel that he probably did a minimum of 3,000. I don't even think he knows how many it was."

Using mountain climbing techniques and uncanny skill in picking locks and bypassing burglar alarms, the 5-foot-3 Kidd worked the Washington area like a man possessed, police said, sometimes burglarizing four or five office buildings a night. Karinshak estimated that Kidd stole more than $1.2 million worth of cash and merchandise over the last three years.

Kidd's May 13 arrest by Arlington police came at a motel on Wilson Boulevard after he paid for his room with traveler's checks stolen the day before from an insurance company in Rosslyn, Karinshak said. Yesterday, meeting with the press hours after his sentencing, Kidd said he was glad he had been caught:

"I was pretty beat by the time I was arrested. I was bored with life. I was shooting up enough drugs to kill other people. I went through some emotional stuff on the streets, so I don't care now."

Kidd said his earliest memories are of an orphanage in Catonsville where he spent most of his days "hooking" school and hiding out in empty box cars parked in a nearby railroad yard.

At 8, he said, he was adopted "by a good family in Gaithersburg. They tried hard, too hard." At first, Kidd said, he was just "what you would call a bad kid." He would sneak away from the house and ride his bike through Gaithersburg, drinking homemade "brew" and apricot brandy, "raising a little hell."

Later it got worse, he said, and he spent time in juvenile homes, "sold dope, sold cars, blew up stuff." When he was 14, he said, he and a friend tried to steal a private plane but failed when he ran it into a fence. Later that year, he said, he fathered a child.

He said he has not seen his adoptive parents since 1970, when they "told me if I wanted to leave they wouldn't stop me . . . I was cut loose pretty young. I never had no authority, no boss. There always comes a time in life when you have to do what you don't want to do, but that time never came for me."

Police said Kidd joined the Marines in 1974 and was trained as a paratrooper and a medic before serving in Vietnam with the 6th Marine Airwing Division. He was honorably discharged in 1978.

Since then, police said, Kidd has been a full-time criminal. Living around the Washington area, sometimes in luxury hotels, sometimes in 14th Street flea-bags, Kidd moved in a world dominated by prosititutes, drugs and fast money.

Kidd said he always took a cab en route to his crimes, carrying a knapsack with a 100-foot rope inside. Hooking to roofs, he usually rappelled down the sides of buildings, entering through unlocked, upper story windows, police said. His targets were most frequently office buildings, but Kidd said he also stole guns and government transportation requests from military installations such as the Army's Aberdeen Proving Ground and Fort Meade.

He also said that, acting on dares, he "penetrated" the FBI's Hoover Building and its Alexandria field office and the CIA headquarters. He claimed to have entered a third floor CIA office and drunk a half bottle of expensive gin. A spokesman for the FBI said yesterday that he knew of no break-ins. A spokesman for the CIA declined to "talk about our security."

Kidd said that his crimes were motivated by a $1,500-a-week addiction to heroin, cocaine and amphetamines -- which he shot with a needle into his right arm. He once went off the drugs for two days, he said, during which time he handcuffed himself to a radiator. "All I could think of was getting a shot. I pulled the radiator off the wall, but I couldn't get out the door."

A self-described loner who had few friends, police said, Kidd often went on spending binges, staying in hotels such as the Capitol Hilton, eating in posh restaurants, sometimes spending up to $5,000 a week. Police said he bought cars for at least four prostitutes.