Gonorrhea has fallen to the lowest level on record in the United States, while the rates of other sexually transmitted diseases — syphilis and chlamydia — are on the rise, federal health officials said Tuesday.

The seemingly paradoxical findings can be explained by the cyclical nature of syphilis outbreaks and a rise in risky sexual behavior among gay men, researchers said.

The nation's gonorrhea rate fell to 113.5 cases per 100,000 people last year, the lowest level since the government started tracking cases in 1941, according to the Centers for Disease Control and Prevention.

At the same time, health official saw increases in syphilis, which is far more rare but has been increasing since 2000. The rate of reported early-stage syphilis was 4.7 cases per 100,000 in 2004, up 81 percent since 2000.

The chlamydia rate rose to 319.6 cases per 100,000 in 2004, up about 6 percent from the year before. But researchers said it is not clear whether the rise represents a real increase in the prevalence of the disease, or simply reflects better awareness and detection.

All three diseases are caused by sexually transmitted bacteria.

London researchers reported earlier this year that because of the life cycle of the syphilis bacteria, infections tend to peak at eight- to 11-year intervals. Sexual behavior affects the overall number of people infected, but regular ups and downs are intrinsic to the disease, the researchers said. Gonorrhea does not follow the same pattern, they said, and rates have been gradually falling since the 1980s.

Dr. Ronald Valdiserri, acting director of the CDC's National Center for HIV, STD and TB Prevention, said that may be part of the explanation for the rebound in syphilis, but a primary reason appears to be an increase in risky sexual behavior.

In 2004, about 64 percent of reported early-stage syphilis infections occurred among men who had sex with men, up from 5 percent in 1999, according to the CDC.

"It's very clear that for the last four years, when we've seen an increase it's primarily been in men and predominantly in men who have sex with men," Valdiserri said. "We know that's being fueled by increases in high-risk sexual behavior. We have good data to substantiate that."

CDC officials are hoping stronger efforts to educate gay men and others about syphilis will help arrest the infection trend, he said.

As for chlamydia, a urine test for men is becoming increasingly common, and health officials have worked to make chlamydia screening routine in yearly gynecological exams for sexually active women.

Chlamydia is the most common of the three diseases. A total of 930,000 cases were reported last year. But health officials believe as many as 2.8 million new cases may actually be occurring each year.

About 330,000 cases of gonorrhea — once known as "the clap" — were reported in 2004.

Syphilis, a potentially deadly disease that first shows up as genital sores, has become relatively rare in the United States, with about 8,000 cases reported in 2004.