Del. Patricia H. Billings (D-Montgomery), a longtime Democratic activist and an abortion-rights advocate in the Maryland House of Delegates, died Saturday at Bethesda Naval Hospital. She had lung cancer.

Billings, 54, who was named in 1989 to fill the seat vacated by the retiring Donald Robertson, won her election bid for the seat representing Chevy Chase and Kensington in a crowded Nov. 6 race. She was to be sworn in Jan. 9, when the legislative session begins.

Her family said yesterday that Billings had fought lung cancer for the past 3 1/2 years. She had a recurrence of the disease during her campaign but kept her illness secret.

"She never wanted her illness to be an issue," said Leon Billings, her husband. He said she went through therapy after the cancer was diagnosed in 1987. In 1988, she was given a clean bill of health, her husband said, and made it through the 1989 session without incident.

"It was late in 1989 when she had a minor reoccurrence," Leon Billings said. "It was after she had announced for the November election she found out that the cancer had come back and had come back in a big way. It is possible that the presence of cancer motivated her more."

She was undergoing chemotherapy while campaigning, her husband said, but still was tireless.

Del. Patricia R. Sher (D-Montgomery), who ran on a ticket with Billings, said yesterday that she was one of the few people who knew Billings was ill. "She was the toughest lady I ever met in my life," Sher said. "She often did not want people to know about her illness. She did not want people to feel sorry for her.

"She is certainly going to be a great loss to all of us. She was a wonderful friend, a good legislator and a fine public servant. The biggest loss particularly is her expertise. She was the state's expert on redistricting," Sher said.

Robertson, the former majority leader of the House, said yesterday, "This is a very tragic occasion . . . . I think it's a great loss for Montgomery County and the state."

Robertson said the process for filling the vacancy begins informally with a caucus of local officials that makes a recommendation to the Montgomery County Democratic Central Committee. The committee, by state law, must send its nomination to the governor within 30 days after the seat is declared vacant. The governor then has 15 days to make an appointment.

Robertson said the committee almost certainly will nominate an abortion-rights advocate. "Pat Billings was strongly pro-choice," he said. "Her position on that issue and the position of the voters on that issue should be and will be respected." More than likely, he said, no one will be named before the legislative session begins.

Patricia Billings was born in Sidney, Mont., on Nov. 9, 1936. She grew up in Glendive, Mont., where she graduated from high school. In 1958, she graduated with a degree in English from the University of Montana. In 1961, she received a master's degree in teaching from San Jose State University in California. She taught journalism in California and in Rockville.

Survivors include her husband, of Silver Spring, and their three children, Shannon, of Rockville, and Paul and Erin, both of Silver Spring; her mother, Mary Harstad of Glendive; and two sisters, Margot Portal of Dijon, France, and Kathie Urbanec of Missoula, Mont.