Katharine F. Lenroot, 90, who was head of the U.S. Children's Bureau from 1934 to 1951 and who helped organize some of the country's major programs for children and mothers, died Feb. 10 at the Colonial Manor nursing home in Milwaukee. She had a heart ailment.

Miss Lenroot, who was born in Superior, Wis., graduated from the University of Wisconsin. She joined the Children's Bureau as a special investigator in 1915 and remained in it until her retirement.

The agency was established in 1913 to report "upon all matters pertaining to the welfare of children and child life among all classes of our people." It was estimated then that 61 mothers died for every 10,000 live births in the United States and that 100 babies died during the first year of life for every 1,000 that were born.

Miss Lenroot was appointed chief of the bureau by President Franklin D. Roosevelt. With the passage of the Social Security Act in 1935, the agency became responsible for providing grants to the states for the development of maternal and child health services, services to crippled children and child welfare services.

In 1938, it became responsible for the child labor provisions of the Fair Labor Standards Act. From 1943 to 1947, it administered the emergency and infant care program for child-bearing wives of members of the armed services. This involved 1.2 million mothers and care for 200,000 children of servicemen who became ill.

By 1949, maternal mortality in the U.S. was down to 8 per 10,000 births and infant mortality was down to 29 per 1,000 births.

When Miss Linroot retired, President Harry S. Truman wrote to her, "The children of this country are better off for your having been in the government. What greater satisfaction could anyone take into retirement?"

Miss Linroot, who had lived in Milwaukee for the last five years, leaves no immediate survivors.