Correction: An earlier version of this article incorrectly said that incidence of HIV infection increased 34 percent among gay men from 2006 to 2009. HIV incidence in gay men ages 13 to 29 rose that amount over that period. This version has been corrected.

The number of people becoming infected with the AIDS virus was about 50,000 a year between 2006 and 2009, according to data compiled by government epidemiologists. That stable incidence, however, masked a steady 12 percent-per-year rise in one high-risk group: young black male homosexuals.

In all, the number of gay black teenagers and men in their 20s becoming infected each year rose by nearly 50 percent, from 4,400 in 2006 to 6,500 in 2009. It was the only group of gay males that showed a significant increase.

“We’re very concerned about this trend,” said Thomas Frieden, director of the Centers for Disease Control and Prevention, whose epidemiologists prepared the report. Of the flat trend in overall HIV incidence, he said, “we’re glad it’s not increasing, but it’s not good enough.”

Studies have shown that young gay black males do not have riskier behavior or more sex partners than their white counterparts. Instead, their much higher chance of becoming infected is a result, in part, because so many already have HIV and don’t know it.

“Knowing your status is critically important,” Frieden said. “It cannot only keep you out of the hospital, it can prevent spreading HIV to others in your community.”

The new report appears in the Public Library of Science journal PLoS One. It uses data from 16 states and two cities (Chicago and Philadelphia) to impute national estimates.

The “latest” national HIV data is almost always several years out of date. Whether the trends described in the report reflect current trends isn’t known.

Using more accurate methods of identifying new — as opposed to long-standing — infections, the researchers estimated that 48,600 people acquired HIV in 2006, 56,000 in 2007, 47,800 in 2008 and 48,100 in 2009.

In all, about 1.2 million people in the United States have the AIDS virus. About 20 percent of them don’t know they are infected, according to epidemiological surveys.

In the latest year, 77 percent of new HIV infections occurred in men, 61 percent in “men who have sex with men” (which is the term epidemiologists prefer for male homosexuals) and 44 percent in African Americans.

New cases were either stable or declining in all categories — men, women, heterosexuals, injection-drug users — with the exception of gay males ages 13 to 29, among whom the incidence increased 34 percent over the four years. Nearly all of that was driven by the increase in black males in that age group.

Minority women — many of them sex partners of men who are bisexual or who use drugs — are also at relatively high risk. Compared with their white counterparts, the rate of infection among black women is 15 times higher, in Hispanic women, it is three times higher.

In recent years, CDC has had a strategy of “high-impact prevention” in which it provides money to state and local health departments for HIV programs and testing, especially targeted at high-risk groups.

“What we’re doing is making sure that the money actually follows the epidemic,” Kevin Fenton, director of HIV prevention at CDC, told reporters in a briefing.

In the past three years, that effort identified 18,000 newly infected people and enrolled about three-quarters of them in treatment, according to an announcement in June.

Daniel C. Montoya of the National Minority AIDS Council, an advocacy group, worries that recent events may erode that effort.

He said that “as Congress works to determine which programs will be slashed to achieve the trillions of dollars in cuts,” he hopes lawmakers “will consider these sobering estimates.”