Syphilis is on the rise in the United States for the first time in more than a decade, largely because of outbreaks among gay and bisexual men in several big cities, the government reported Thursday.

The trend suggests a potential resurgence in transmission of the AIDS virus, the Centers for Disease Control and Prevention said.

The CDC report followed government warnings earlier this year that gay and bisexual men may be letting down their guard against sexually transmitted diseases.

"We are very, very concerned about what the future might hold if these trends continue to go in an upswing," the CDC's Dr. Ronald Valdiserri said.

The syphilis rate increased from 2.1 cases per 100,000 people in 2000 to 2.2 cases per 100,000 last year, the CDC said. The rate had been dropping every year since 1990, and the 2000 rate was the lowest since 1941, when nationwide reporting of the disease began.

Syphilis among women actually dropped 17.6 percent in 2001. More than two-thirds of the new syphilis patients were men.

Although the actual increase in cases was small — 124 more people, for a total of 6,103 new cases — the rise among gay and bisexual men has caused concern that public health safeguards put in place over the last two decades during the AIDS epidemic may be crumbling.

Since 1997, syphilis outbreaks have erupted in New York City, Seattle, Chicago, San Francisco and Miami.

"Our challenge is to underscore the connection between HIV and syphilis and renew the commitment these groups brought in the early years of the HIV epidemic," Valdiserri said.

Officials said the problems include a new generation that does not remember the devastation of the AIDS epidemic, lack of concern because of life-extending AIDS-treatment drugs, and burnout after years of good-health messages and safe-sex practices.