More than 44 million Americans were obese and 16.7 million people had diagnosed diabetes in 2001, according to the study by the Centers for Disease Control and Prevention.
The researchers found the nation's obesity rate climbed to 20.9 percent in 2001 from 19.8 percent the year before, and the rate of diagnosed diabetes rose to 7.9 percent from 7.3 percent.
The study appears in Wednesday's Journal of the American Medical Association.
"These two rates are alarming. They have a lot of implications on public health in this country," said CDC epidemiologist Ali Mokdad.
Obesity is associated with a variety of illnesses, including diabetes and high blood pressure.
The study's findings are based on telephone surveys with a nationally representative sample of 195,005 adults.
The study used self-reported data to calculate body-mass index, a height-to-weight ratio. A BMI of 30 or higher was considered obese in the study.
Researchers believe the real rates are even higher, partly because people tend to underestimate their weight when asked.
The study confirmed previous findings that Mississippi is the state with the highest rate of obesity and Colorado the lowest. The highest rate of diagnosed diabetes was in Alabama; the lowest was in Minnesota.