About 40 relatives of American hostages being held in Iran attended a Christmas service yesterday at the Washington Cathedral where they were read a letter from Bruce Laingen, U.S. charge d'affaires in Tehran.

Reading from the letter, the Right Rev. John T. Walker expressed Laingen's hope for "reconciliation and brotherhood" between Moslems and Christians.

Walker, who officiated at the hour-long service, also called attention to the prayers and expressions of support the American people have offered for the hostages.

The families of 10 hostages attended the service, including the Laingen family. Laingen is not a hostage at the embassy, but he has been detained at the Iranian Foreign Ministry in Tehran since the embassy takeover.

"May that light of Christmas and that promise of peace strengthen each of us to face every challenge with courage and every disappointment with hope," Laingen's letter said.

"We extend to you our grateful thanks for your support, your good will and your prayers -- above all for your cathedral's vigil prayer for the ultimate triumph of reconciliation and brotherhood that is the promise of both the Moslem and Christian faiths," the letter continued.

Louisa Kennedy, wife of hostage Morehead C. Kennedy, said later she was pleased that American clergymen had been able to visit the hostages for Christmas.

"I'm sure they had a very meaningful service last night at the compound," she said, "and I'm sure they were all with us here today in spirit."