Following in the footsteps of his grandfather and uncle, Dr. Frank S. Preston went into the medical field to make life better for the sick.

He opened his first private practice in 1963 at 25th and Riverside Avenues in Minneapolis across the street from the former St. Mary's Hospital, opened by his grandfather, Horatio Sweetser, who became its first chief of staff. Preston, whose specialty was internal medicine and pulmonary disease, served as chief of staff at St. Mary's in the 1980s.

"He was held with highest esteem by fellow physicians and patients," said Joseph Hamel, a retired doctor who worked with Preston at St. Mary's. "He was a man with great integrity."

Preston died of lung cancer Saturday at Fairview Southdale Hospital in Edina. He was 81.

He became an Eagle Scout at age 16 while attending DeLaSalle High School in his native Minneapolis. In the 1940s, he studied pre-med at the University of St. Thomas in St. Paul and was captain of the swimming team. He earned his bachelor's and doctorate at the University of Minnesota.

After serving as a flight surgeon with the Air Force's Northeast Air Command in Greenland, Preston completed an internship at St. Vincent's Hospital in New York City and practiced internal medicine at the Minneapolis Veterans Medical Center, said his granddaughter Rachel Drewelow, of Austin, Minn.

He worked for a couple of years in a clinic at 38th Street and Nicollet Avenue S. before opening his own practice and developing a relationship with St. Mary's, which later merged with the Fairview and University of Minnesota systems. He was responsible for starting the pulmonary function laboratory at St. Mary's, said his daughter Monica Drewelow, of Minneapolis.

"He loved his work," said his wife of 52 years, Faye. "Patients loved him because he spent time with them."

Sometimes he offered free services to those who could not pay, family members said.

Preston was a chest consultant for the University of Minnesota Health Service from 1963 to 1972, and a clinical professor from the 1960s to the 1990s. He ended his career in 2001, retiring from the Fairview Oxboro Clinic in Bloomington.

An avid swimmer, he won several medals in the Senior Olympics. He also was an outspoken critic of embryonic stem cell research and in later life made presentations at the State Capitol and wrote letters to the editor that appeared in the Star Tribune and national publications such as the USA Today, his granddaughter said.

Besides his wife and daughter Monica, Preston is survived by three other daughters, Martha Preston, Mary Preston and Margaret Preston, all of Minneapolis; five sons, Bill, Tim, John and Dan, all of Minneapolis, and Frank, of Denver; a sister, Margaret Preston, of St. Paul; a brother, Thomas, of Omaha, and six grandchildren.

Services will be held at 11 a.m. Friday at the Church of Christ the King, 5029 Zenith Av. S., Minneapolis. Visitation will begin one hour before services at the church.