Irene Blekas says she felt safe in her Arlington apartment for 10 years. But recently, her son's bike was stolen from her garage.

"There seems to be a lot more crime than I used to hear about, but this was the first time it hit me," she said.

Blekas is not alone, according to statistics on serious crimes in the county for the first nine months of this year. While the total number of serious crimes fell slightly from the first nine months of 1983 to the same period this year, burglaries and car thefts rose, and the number of aggravated assaults, which jumped 40 percent from 1982 to 1983, continued to rise.

The drop in serious crimes -- including homicide, rape, aggravated assault, burglary, larceny and vehicle theft -- follows a slowing trend nationally and in the Washington area over several years.

Arlington was alone among area jurisdictions in showing an increase in the number of burglaries in the first half of this year as compared with the first half of 1983. New nine-month crime figures were not available yesterday for all jurisdictions.

County police say they have no explanation for the swelling burglary rate, but note that mild weather, which encourages homeowners to leave doors and windows open, may contribute to the increase.

The number of serious crimes fell 1.3 percent, from 5,640 from January through September of 1983 to 5,565 in the same period this year. After a 17 percent decrease in serious crimes in the county from 1981 to 1982, that number dropped 4 percent from 1982 to 1983.

The number of rapes, robberies and larcenies fell from the first nine months of last year to the same period this year. The number of homicides remained the same at five.

Aggravated assaults rose 13 percent, from 253 in the first nine months of 1983 to 286 in that period this year. And vehicle thefts rose a substantial 24.9 percent, from 342 in 1983 to 427 in 1984. Burglaries rose about 13 percent, from 949 through September of last year to 1,065 so far this year.

Sgt. John Blake of the Arlington burglary unit said the crimes tend to occur in cycles. "Realistically, burglaries can't go down forever," he said.

Blake said he advises homeowners to keep their houses and apartments well-lit, ask neighbors to watch their property when they are away, record the serial numbers of appliances, photograph distinctive pieces of jewelry and, finally, "have good locks and use them."

While police say they are not anxious about the rise in burglaries, the increase has caused worry by some Arlington residents, and booming business for local locksmiths.

Vincent Smith, manager of AAA Locksmith Service at 2401 Columbia Pike, said his shop's business has jumped about 25 percent since the spring. "We're busier than ever," Smith said. "We find people installing locks before they even have an incident."

Some Arlington residents said the rise in burglaries has made them more careful to use their locks.

Since Cathy Lutz moved to North Fifth Street seven months ago, the ground-floor apartment downstairs has been broken into five times. "I only have one door, so I keep it locked even when I'm there," Lutz said.