As metropolitan Washington enters the 1980s, it does not have many more people that it did a decade before. It grew from a little less than 3 million to a little more than 3 million. But the Washingtonians of today are quite different in many respects from those of 1970.

One preschool child population is now more than one-fourth smaller than it was at the beginning of the 1970s. At last count, it was still decreasing. The elderly among us have increased in number, but relatively little compared with some other age groups. The much more dramatic growth has been among young adults. The 25-34-year age bracket increased by more than 116,000 persons in the 1970s, which was over two-thirds of the net increase among Washingtonians of all ages.

The national "singles boom" is reflected in today's Washingtonians -- amplified to some extent by the area's exceptionally large population of young adults. The number of persons in this area who have never been married has increased by about one-third since 1970. The divorced population has grown by over 50 percent. At the same time, the married population has hardly increased at all. Married households are still ahead with a total of about 600,000, but their margin has declined considerably.

The Washington area's population of races other than white or black, meaning mainly Asians, tripled between 1970 and 1977. Recent statistics equivalent to those for racial groups are not available for Hispanics, who are an ethnic group rather than a separate racial category. But public school data show that the number of Hispanic children enrolled increased by 75 percent between 1970 and 1978.

The area has also become less racially concentrated than before. The number of blacks living in the Washington suburbs more than doubled in the first seven years of the last decade, and the 1980 census may well show that more than half of the area's blakcs have become suburbanites.

Concurrently, the racial composition of the city has stablized after becoming increasingly black for decades, and the number of upper-income households has begun recently to grow at a more rapid rate in the District than in the suburbs.

Today, the most rapidly growing household category by far consists entirely or mainly of adults. Not only are there many more single persons than before, but more of the marrieds are childless. Households with only one or two persons constitute more than a 52 percent majority of all in the area, and the size of the average area household has dropped below 2.8 persons.

Families with children have by no means disappeared, of course. The number of area households containing at least one child grew slightly between 1970 and 1977 -- a little less than 5,000, or about 1 percent. But they are increasingly outnumbered by those without children at all. In fact, 97 percent of the net increase among area households from 1970 to 1977 are accounted for by childless households. Households without children now exceed 60 percent of all households in the area. Households with only one child now make up two-fifths of all households with children.

Nor are most childless couples young adults. Couples past 45 who do not have children living with them outnumber younger childless couples by nearly 50 percent.

What are the new adult households -- married and single -- like? Almost half of all single adults who live alone have passed their 45th birthdays. The over-45s are in the majority among single women.

Among the new minorities, the Asians more than tripled between 1970 and 1977. At the beginning of the 1980s, they almost certainly number more than 100,000 -- making up between 3 and 4 percent of all people in the area.

In terms of incomes, both Asian and Hispanic households appear to be doing better than blacks in this area, though not as well as non-Hispanic whites. Educational levels are higher than among the white majority.

The new urbanites are another pouplation group that has emerged as significant. The are people who are moving into the revitalizing central city:

Previous residence -- About 55 percent of the newcomer households heads came from outside the Washington area. The other 45 percent came from the Washington suburbs.

Age -- About two-thirds of the household heads are less than 35 years of age.

Race -- Two-thirds of the newcomer households are white; 28 percent are black; the remaining 5 percent are of other races.

Household size -- Four-fifths have only one or two persons.

Tenure -- Most of the newcomers who moved to the District in the year prior to the survey rent their dwellings -- nearly 89 percent. As length of residence increases, however, other data show that a larger proportion own their dwellings.

Income -- More than two out of five newcomer households had incomes of $15,000 or over in 1977. About 3,300, or one-fifth, had incomes of $25,000 or more.

If there is one thing that can be said about the 1980s with some certainty, it is that they are unlikely to be tension-free. The upward stretching of the income distribution, which has resulted in an increasing number of high-income households while the number with low incomes has remained virtually the same, is bringing the area somewhat closer to a kind of "two-class society," rich and poor.

Especially in the District, where middle-class black families have been leaving for the suburbs, a "hole-in-the-middle" effect is developing. A sense of threat is certainly present today among many less fortunate residents of the inner city, especially those who fear displacement by the new urbanites who are rehabilitating houses around them.

Racial disparities are also present as an element here, further complicating the situation. The area's poor households are by no means all black, but blacks are prominent among them. By the same token, while the new urbanites are both white and black, most are white, and the whites are more noticeable. What is involved may well be more a transition of class than race, but the racial component sharpens and focuses fears and resentments.

Some of the same potential is presented by the rapid increases and obvious success of the new minorities, especially those from Asia. They are moving with skill, ease and assurance into ownership of management of many of the area's small businesses -- including convenience stores and filling stations, to name just two kinds. Their obvious success in these enterprises offers a tangible reminder to low-income residents who grew up in the area of their own less hopeful situation. At the same time, illegal aliens, many of them Hispanic in origin, have been taking a growing number of the lower-skilled jobs that my be the best these low-income Washingtonians would otherwide get.

The human relations problems that confront this area as it enters the 1980s are therefore different in mahy ways from those of a couple of decades ago, but they are no less challenging.