Most came to honor those who had fallen in combat, but Bahar Hess came to Arlington National Cemetery yesterday to remember someone who may be dead or alive - she doesn't know which.

It was the second year that Hess has joined others at the cemetery for an informal service honoring the Vietnam war dead and the more than 600 men who, like Hess's husband, remain classified as missing in action from that war.

Yesterday there were only 20 people gathered on a hilltop in the cemetery, to sing two folk songs and remember the men whom many of the children attending the service say they never got a chance to know. It was the smallest of three ceremonies in the cemetery yesterday and in many ways one of the more poignant.

"We are having a remembrance ceremony today to remember our fathers .. . We don't want to forget them," said 15-year-old Heather Schott as she led a group of children in the Vietnam ceremony.

Along with 11 other children of Vietnam servicemen, she placed a single red rose on the graves of victims of the war.

Earlier about 50 people gathered at the grave of former president John F. Kennedy as an army major general laid a wreath on the president's grave. Yesterday was Kennedy's birthday and he would have been 61.

The only notable figure at the Kennedy grave was Evelyn Lincoln, his private secretary in the White House, an army spokeman said.

Later about 1,500 people gathered in the cemetery's amphitheater as Defense Secretary Harold Brown, speaking on behalf of President Carter, gave the formal Memorial Day speech. He called for a "quiet and firm strenth based on adequate armed forces, solid and determined leadership and cohesion among our people."

For some, the Brown speech, the bands, and the 19-gun salute were enough to mark the holiday. "It speaks to me of some past values that we, to some extent, have lost, but not entirely," said Gerald Vaughn, a blind man from Washington.

At the ceremony in the amphitheatre a woman vocalist from the Air Force who sang "The Battle Hymn of the Republic." "Ladies and Gentleman, you're witnessing a first," announced Robert E. Sutphin, president of the Grand Army of the Republic, a veterans group that sponsors the program."That is the first time we've had the fairer side of the sexes be a soloist out here," he said.

As the hilltop service staged by the Vietnam widows and their children ended, Bahar Hess, whose husband has been missing in action in Vietnam since March 29, 1969, said she was glad she had come. "It's very nice that the men are remembered like this." she said.

Then, she added, there are moments like yesterday when, "I stop and think, is my husband alive?"