Although the death rate in the United States fell 8 percent in seven years, the mortality from lung cancer and other smoking-related lung diseases keeps going up, federal researchers said.

The nation recorded about 2.1 million deaths in 1986, or 873 for every 100,000 Americans, down 8 percent from 1979, when the rate was approximately 950 per 100,000, researchers at the Centers for Disease Control said.The decline, the CDC said, is partially because of decreases in the death rates from coronary heart disease - the nation's leading cause of death, accounting for about 593,000 fatalities a year - and stroke, which kills 125,000 people a year.

Mortality from heart disease fell 18 percent to a rate of 227 per 100,000 in 1986, and the rate from stroke dropped 26 percent, to 56 per 100,000.

Heart disease and stroke are two illnesses for which smoking is considered a risk factor. But although mortality rates from those illnesses were declining, death rates from two other smoking-related causes of death - lung cancer and other smoking-related lung diseases - were on the rise.

The mortality rate from lung cancer, which kills 126,000 Americans each year, rose 15 percent from 1979 to 1986, to 50 deaths per 100,000 people. And the rate for smoking-related lung diseases - such as bronchitis and emphysema - rose 33 percent, to 28 per 100,000. Those diseases kill 71,000 Americans a year.

The CDC said that smoking "is a stronger risk factor" for lung cancer and smokers' lung disease than it is for heart disease or stroke.

The trends, the CDC noted, occurred while smoking in the United States was going down - from 34 percent of the population in 1979 to 29 percent in 1987, the latest year for which those figures are available. Mortality from lung cancer and smoking-related lung disease kept increasing as they preyed upon smokers from many years ago.