The study tracked customers at McDonald's, Wendy's, Burger King and Kentucky Fried Chicken in poor New York City neighborhoods with high obesity rates.
Half of customers noticed calorie counts. Twenty-eight percent of those said postings had influenced what they ordered. Nine out of 10 of those said they'd made healthier choices as a result.
But New York University and Yale researchers say receipts showed people had ordered slightly more calories than the typical customer before the law went into effect in July 2008.
The study will be published Tuesday in Health Affairs.
"I think it does show us that labels are not enough," Brian Elbel, an assistant professor at the New York University School of Medicine and the lead author of the study, said in an interview with the New York Times.
This study focused primarily on poor black and Hispanic fast-food customers in the South Bronx, central Brooklyn, Harlem, Washington Heights and the Rockaways in Queens, and used a similar population in Newark, which does not have a calorie posting law, as a control group, according to the newspaper.