Bob Backus, who used weight training to help overcome serious illness and became the world's best weight thrower, died on June 30 at Vencor Hospital in Boston. He was 72.

Backus had been in poor health for several years and had suffered a stroke last December, his lawyer, J. Marlin Hawthorne, said.

In 1948, Backus, at 6 feet 5 inches and 160 pounds, took up weight lifting to help recover from spinal meningitis. During World War II, he was so thin that the Army Air Forces dropped him from its cadet program. In those days, most athletes feared that lifting weights to gain strength would make them muscle bound. Backus disagreed and built himself into a 290-pound strongman.

At Tufts University, he started a lifelong dedication to weight throwing. The track and field musclemen who threw the 16-pound hammer outdoors often threw the 35-pound weight indoors and the 56-pound weight (no longer used) outdoors. The weights were iron balls connected by short steel links to a rigid triangular handle.

Backus set career bests of 45 feet 2 inches in 1957 for the 56-pound weight and 66-2 3/4 in 1959 for the 35-pound weight. Both performances were American records, in effect world records because the weight throws were contested mainly in the United States.

In those days, the throwing circle had no warning rim, just a painted line, and there were no shoes for throwers. Backus solved that problem by wearing ballet slippers.

He won seven consecutive American titles in the 56-pound weight throw (1953-59), seven in eight years in the 35-pound weight throw (1954-61) and one (1954) in the hammer throw. He won a gold in the hammer throw in the 1955 Pan American Games and in 1982 was voted the best indoor weight thrower ever.

In his prime, he competed almost weekly in New York and Boston against Al Hall, Stewart Thomson and Tom Pagani, his New York Athletic Club teammates, and sometimes against Harold Connolly, the 1956 Olympic hammer-throw champion. During the winter, because the weights would have destroyed arena floors, the competition was in cold armories, such as the Squadron A Armory on the East Side of Manhattan.

Before dawn, the athletes would load their families into cars and drive 200 or 300 miles to compete. Sometimes, they persuaded officials to hold one meet right after another so they could compete twice.

Joe McCluskey was then the New York A.C. coach. ''Bob Backus loved to compete,'' the 88-year-old McCluskey recalled yesterday. ''They would compete so hard among themselves, and then they would go out with their families together.''

Backus lived in Pembroke, Mass., and for years owned a gymnasium there. He was a fitness consultant whose clients included the Yankees during 1983 spring training.

In the 1952 Helsinki Olympics, he finished 13th in the hammer throw. While there, he met Elsa Torikka of Finland, who placed 17th in the women's javelin. They were married in 1957 and later divorced. He is survived by his former wife and a daughter, Lisa, of Skaneateles, N.Y., and a son, Niles, of Honolulu.

Until his health declined, Backus remained active. At 51, he finished second in the 35-pound weight throw in the national indoor championships. He was amused at the fuss.

''People overdo this age thing,'' he said, ''but I guess you could say this was a victory for the Geritol set.''