That is a decline from 21.6 percent in 2003 and 22.5 percent in 2002, the Centers for Disease Control and Prevention reported Thursday.
The rate has fallen steadily since the late 1990s. The fall from 2002 to 2004 was the largest two-year drop since the late 1980s, public health advocates noted.
Increased cigarette taxes, workplace smoking bans and state-based prevention efforts are the main reasons for the decline, said Dr. Corinne Husten, acting director of the CDC's Office on Smoking and Health.
At the same time, officials said it appears increasingly unlikely the nation will reach the public health goal of reducing the smoking rate to 12 percent by 2010.
The results are based on a national household survey of 31,326 adults. People were defined as current smokers if they had smoked at least 100 cigarettes in their lifetime and said they still smoked on a daily or occasional basis.
Because the margin of error was plus or minus 0.6 percentage points, Husten said the difference between the 2004 and 2003 rates was not statistically significant, but the gap between the 2004 and 2002 rates clearly was.
The survey found smoking rates were highest among American Indians and Alaska Natives. They also were higher among men than women, higher among people living below the poverty level, and higher among people with no more than a high school degree than among those with graduate degrees.
The highest smoking prevalence was reported in Kentucky (27.6 percent), West Virginia (26.9 percent) and Oklahoma (26.1 percent). The lowest rates were in Utah (10.5 percent), California (14.8 percent) and Idaho (17.5 percent).
By Mike Stobbe