More than one-fourth of sexually active inner-city girls were infected with chlamydia, a disease that usually has no symptoms but often causes infertility, a study found.

Researchers say sexually active girls should be tested for chla-mydia every six months, twice as often as current guidelines recommend. Chlamydia cases are usually easily curable with antibiotics.The bacterial disease is the nation's most commonly reported infection, with 4 million new cases a year and a cost estimated at $2.4 billion. Sexually active adolescent girls have the highest risk.

Chlamydia is the leading preventable cause of pelvic inflammatory disease, an often painless infection that can result in internal scarring. Infertility or tubal pregnancy occur after 1 in every 10 chlamydia cases.

In the study, the highest chlamydia infection rate - 27.5 percent - was among 14-year-olds, the researchers reported in today's Journal of the American Medical Association.

With many girls being tested more than once in the 1994-96 study, the overall positive test rate was 29.1 percent. Subjects were 3,202 sexually active females ages 12 through 19 at Baltimore clinics that specialized in family planning or sexual disease or were based in schools.