William T. Hannan, 73, a Washington attorney and Catholic lay leader whose professional concerns and private interests involved him in a variety of causes, died of a stroke July 8 at Sibley Memorial Hospital.

A fourth generation Washingtonian, Mr. Hannan was senior partner of O'Connor and Hannan, a firm he helped organize in 1973 that now numbers more than 90 attorneys in four offices in this country and abroad. From 1951 to 1973, he had been a partner of Hannan, Castiello & Berlow.

One of his principal legal interests was the redevelopment of urban properties. A recent legal involvement was his service as counsel for Capitol Gateway Corp., a firm that sought to develop a 50-acre tract south of the Capitol.

In 1960, Mr. Hannan was a figure of controversy when he was cited as the chief architect of House-passed legislation to curb the activities of the D.C. Redevelopment Land Agency. Opponents of the bill said it would be harmful to urban renewal. Mr. Hannan rejected this criticism, asserting that "urban renewal is a wonderful lifesaver for cities. It's a shame it isn't being done."

A graduate of St. John's College High School, Mr. Hannan earned his law degree in 1936 from Catholic University. After passing the D.C. bar examination, he joined the firm of Canfield and Quinn.

Once, after settling a case for a radio company, he was invited to a restaurant to celebrate. During the festivities he rose to sing with a musical ensemble. Impressed with his Irish lilt, the radio men offered him a job singing on their stations, which later became part of the Mutual Broadcasting System. Mr. Hannan declined, but he continued to sing at private gatherings.

As a Catholic layman, Mr. Hannan served as the first chairman of the Archdiocesan Charity Campaign in which more than $1 million was raised. He was a major contributor to the National Children's Choir, an organization whose members represented a variety of Washington's racial, cultural and religious groups.

He also was a founding member of the John Carroll Society, a member of the board of the National Conference of Christians and Jews and a trustee of Catholic University and Trinity and Marymount colleges. He lived in Washington and was a member of the parish of Our Lady of Victory Catholic Church.

In 1970 he was elected president of the Washington Heart Association, where he had been a board member for 20 years.

In the mid-1970s, Mr. Hannan was active in raising funds for refugees from Southeast Asia. In addition to supporting a Cambodian family, he and his wife of 45 years, Jane, gave accommodations to a Vietnamese family for several months in their Washington home.

As an activist in Irish-American affairs, Mr. Hannan was one of the organizers of the annual St. Patrick's Day Parade in Washington. One year, he served as the grand marshal, riding in a horse-drawn carriage and followed by a pack of leashed Irish wolfhounds.

In addition to his wife, of Washington, survivors include two sons, William T. Jr., of Bethesda, and Gregory, of Washington; four brothers, Archbishop Philip M. Hannan of the Catholic Archdiocese of New Orleans, and Thomas K., Denis T. and Patrick J., all of Washington, and five grandchildren.