Lung cancer is expected to kill more than 159,000 Americans this year, nearly 70,500 of them women. So even a small improvement in survival is welcome, and can add up over time, said Ward.

Smoking became rampant among men long before women, and thus men's lung cancer deaths soared first. But in the early 1990s, death rates began dropping among men as older smokers died and fewer younger men took up the habit. Those rates were dropping 3 percent a year between 2005 and 2007, the new report said.

Researchers had long anticipated the same pattern would appear among women, and had been tracking signs that women's death rates had begun inching down for a few years. But only now with a solid five-year trend are they confident that the decline is real, said National Cancer Institute statistician Brenda Edwards, a report co-author.

But Edwards noted that the cigarette industry targeted advertising toward women in late 1960s and '70s, what she calls "the Virginia Slims effect" that boosted smoking among young women at the time. It's possible that those women will temporarily bump up the death count again as they age, she said.

Perhaps more troubling is that progress at getting men and women to kick the habit has stalled in the past decade.

The news is "encouraging, but we have to be cautious," said Dr. V. Craig Jordan of Georgetown University's Lombardi Comprehensive Cancer Center.

As Congress considers how much money to budget for medical research this year and next, Jordan worried that cutting investments in cancer research and tobacco control could reverse hard-won gains.

"Like all battles, you just let up a little bit and it's all over," he said.