Einar Gustafson, who plucked heartstrings as a 12-year-old cancer patient identified as Jimmy on a national radio program and then vanished for a half-century, only to reappear as a champion of what had become the Jimmy Fund, died of a stroke Sunday at a hospital in Caribou, Me. He was 65.

Though most people assumed he had died as a youth, Mr. Gustafson, who became a grandfather and long-distance truck driver, maintained homes in New Sweden, Me., and Buzzards Bay, Mass. In his final years, he became an active part-time spokesman for the Jimmy Fund, a children's cancer foundation that was first the official charity of the Boston Braves, who moved to Milwaukee and then to Atlanta, and since 1953 of the Boston Red Sox.

The Jimmy Fund has raised more than $150 million, underwritten some of the more significant work on chemotherapy for children and helped reduce the death rate of some childhood cancers -- particularly leukemia -- from 90 percent to 10 percent.

Dr. Edward J. Benz Jr., president of the Dana-Farber Cancer Institute, which is supported by the Jimmy Fund, said Mr. Gustafson's story ''is the story of our nation's war on cancer, and tens of thousands of people have rallied against cancer in his name.''