Despite a worldwide campaign for circumcision to slow the spread of AIDS, the rate of circumcision among American baby boys appears to be declining.

A little-noted presentation by a federal health researcher last month at the International AIDS Conference in Vienna suggested that the rate had fallen precipitously  to fewer than half of all boys born in conventional hospitals from 2006 to 2009, from about two-thirds through the 1980s and ’90s.

Last week, officials at the Centers for Disease Control and Prevention cautioned that the figures in the presentation were not definitive. But they are already stirring a sharp debate on the Internet.

The numbers were presented to the AIDS conference by a C.D.C. researcher, Charbel E. El Bcheraoui. The presentation was not covered by any mainstream news outlets, but a report by the news service Elsevier Global Medical News, along with a photograph of a slide from the presentation, quickly made the rounds of the blogosphere.