According to an annual report on sexually transmitted diseases (STD), by the U.S. Centers for Disease Control and Prevention (CDC), the number of U.S. syphilis cases rose for the fourth year in 2004, and that increase was mainly among men.
Meanwhile federal health researchers say the gonorrhea disease rate reached a historic low.
A rise was also apparently seen in chlamydia, a bacterial infection that can cause infertility in women, but the increase is believed to reflect better screening rather than a rise in new infections.
It seems that syphilis is rarer in the United States than gonorrhea and chlamydia, and health officials believe the rise was primarily among men who have sex with men.
If left untreated syphilis can cause insanity and death.
In a separate CDC analysis it was estimated that men with male sex partners accounted for 64 percent of early-stage syphilis cases in 2004, up from just 5 percent in 1999.
Cities with large gay populations had some of the highest rises in syphilis cases, and San Francisco was top of the league.
Ronald Valdiserri, acting director of the CDC's HIV, STD and Tuberculosis prevention programs, says there is evidence that some men are engaging in higher risk behaviours.
The data however also showed that syphilis declined among women and blacks, previously the worst affected groups.
The gonorrhea rate fell 1.5 percent from 2003 and is the lowest level on record since reporting began in 1941, but there is some concern that the disease is under-reported, and the higher rates among ethnic minorities and rising resistance to antibiotics is thought to be hampering treatment and prevention.
If left untreated, gonorrhea can cause infertility in men and a serious infection of the reproductive tract in women.
The CDC says that STDs, which are spread through vaginal, oral and anal sex, result in medical costs of $13 billion annually and can increase the risk of contracting HIV, the virus that causes AIDS.