The Roman Catholic Church in the United States last year experienced its greatest growth in recent years.

An increase of 443,880 members brought membership of the church in this country to a new high of 49,325,752 Catholics. Most of the increase came from within the Church itself in the form of infant baptisms.

In contrast to most Protestant bodies, which count membership as beginning at young adulthood, Catholics, for statistical purposes, consider baptized infants as members of the church.

There were 79,627 converts to Catholicism last year, 408 fewer than in the previous year.

Statistics on the priesthood recorder in the new Official Catholic Directory, out this week, portend serious trouble ahead for the church.

There are 1,591 fewer priests than there were 10 years ago to serve a Catholic population that has grown by nearly 2.5 million members in the past decade. The dramatic decline in the number of students preparing for the priesthood is even more alarming.

According to the new directory, there are only 15,943 seminarians today, compared to 45,379 a decade ago. This year's enrollment in the seminaries reflects a drop of 1,304 from the previous year.

The statistics bear out a warning issued last November by Apostolic Delegate Jean Jadot that the American church is in serious trouble over the growing shortage of priests.

Catholic schools and enrollment in them continue to decline in numbers.

Total enrollment in Catholic elementary and high schools stands at 3,368,291 students. This represents a drop of 104,410 pupils in the past year, but a loss of 2,293,837 students since 1965, the high point of Catholic school enrollment.

This decline in parochial school enrollment represents both a reason for and a cause for continuing concern over the declining seminary enrollment, since parochial schools are a major source of church vocations.

The number of nuns - 130,804 - has held relatively steady from the previous year but is down 65,867 from the total of 10 years ago.

The number of parishes - 18,572 - increased slightly from 1976 as did the number if bishops now at an all-time high at 283. That total includes active as well as retired prelates.

Locally, the Washington Archdiocese gained 126 new members over the previous year for a total of 396,537; the Diocese of Arlington reported an increase of 5,435 for a total to 150,235.

The Baltimore Archdiocese showed an increase of 51,538 members for a total of 505,952.

Ed Sullivan, research director for the independent Catholic-oriented Center for Applied Research in the Apostolate, pointed out that the annual statistical study, while valuable overall, represents "kind of ball-park statistics" on the church.

"We regard them as a good overall estimate," he said.