The number of serious crimes reported in Arlington County rose 2.4 percent from 1984 to 1985, reversing a trend of declining crime rates in recent years, according to statistics released yesterday by police.

Police said the increase in serious crimes -- murder, rape, robbery, aggravated assault, burglary, larceny and vehicle theft -- from 7,599 to 7,781 in part may be a result of growth in the county and reflects regular cycles in the crime rate. Among the seven categories, only aggravated assaults dropped from 1984 to 1985.

During the last four years, the reported crime rate in the county dropped, with the decreases becoming more slight each year. From 1981-82, serious crime fell 17 percent; from 1982-83, it fell 4 percent and from 1983-84 just 0.2 percent.

"During those years when we had those big drops, I kept saying we would be seeing a leveling-out point. I said there would be an increase as we had growth in the county," Police Chief William K. Stover said. "I'm still not pleased with the level of crime in Arlington, and I doubt if I ever will be."

One reason for reported crime drops early in the decade, police have said, is a decrease in the number of people in the 15-to-25 age group, considered most likely to commit crimes. As the "baby boomlet" begins to swell the ranks of that age group, crime may rise, officials say.

Among the crimes that increased in Arlington were robberies, up 19.4 percent from 283 in 1984 to 338 last year. "Mostly, we're talking about an increase in street and commercial robberies," said Sgt. Frank Hawkins of the homicide and robbery division.

Hawkins added that robberies, which typically decline in the summer and rise before the holidays, remained relatively constant throughout the year. Rapid construction in some areas of the county may help explain the increase because of "the commotion and confusion it brings as well as all the people," Hawkins said, adding that officials rarely can explain short-term trends in the crime rate.

"I wish I knew," he said.

Vehicle thefts, which rose 14 percent from 1984 to 1985, have been a growing problem not just in Arlington, but in other jurisdictions, Stover said.

"We've been watching them for the last two years, and vehicle thefts have steadily increased throughout the region . . . it's getting to be a monumental problem. Thieves aren't taking the $5,000-$10,000 cars; they're taking the high-dollar cars," he said.

While reported crime rose, the number of fatal motor vehicle accidents dropped from 18 to five, and the number of accidents causing injuries from 1,231 to 1,182 -- decreases police said may be a result of campaigns and legislation aimed at curbing drunk driving.