NEW HAVEN, CONN -- John E. Boswell, 47, a former chairman of Yale University's history department who theorized that homosexual marriages were celebrated liturgically in the Middle Ages, died Dec. 23 at the Yale infirmary here. He had AIDS.

In June, Dr. Boswell provoked debate with his book "Same-Sex Unions in Premodern Europe," based on the study of more than 60 manuscripts from the 8th to the 16th century.

By the 12th century, he wrote, the ceremony of same-sex union was "unmistakably a voluntary, emotional union of two persons," one that was "closely related" to heterosexual marriage "no matter how much some readers may be discomforted by this."

Some scholars and theologians have disputed his findings.

James Brundage, a professor of history and law at the University of Kansas, said last summer that "the mainstream reaction was that he raised some interesting questions but hadn't proved his case."

Dr. Boswell gained wide notice in 1980 with the publication of "Christianity, Social Tolerance and Homosexuality: Gay People in Western Europe From the Beginning of the Christian Era to the 14th Century." The book won the American Book Award for history in 1981.

One major aim, Dr. Boswell wrote, was "to rebut the common idea that religious belief -- Christian or other -- has been the cause of intolerance in regard to gay people."

Colleagues said Dr. Boswell was influential and a role model without being an activist.

The Rev. Anita Bradshaw, director of supervised ministry for Yale Divinity School, said he gave the gay and lesbian community a chance to rediscover its own history.

"He was a spokesman and original thinker," said Gaddis Smith, a fellow history professor. "He was much more influenced by his writing than by any call to activism."

Dr. Boswell, a Boston native, was a graduate of the College of William and Mary. He received master's and doctoral degrees from Harvard University. He read or spoke 17 ancient and modern languages.

He joined the Yale faculty in 1975 as an assistant professor and was appointed a full professor in 1982. He was named the A. Whitney Griswold Professor of History in 1990, when he began a two-year term as department chairman. In 1987, he helped organize the Lesbian and Gay Studies Center at Yale.

Survivors include his parents, two brothers and a sister.