F.D.R. has been researched now for more than twenty years, from Lerner to Leuchtenberg, and it is surprising that major sources of untapped primary material still remain. The bond between the Hudson River Brahmin and the immigrant from Austria became one of the President's firmest friendships, and Frankfurter was his confidant at some critical moments. Close reading of the letters reveals Frankfurter's pivotal role in the court-packing struggle of 1937. The letters are best during the ""high"" New Deal years of '33-38, when Frankfurter was at the Harvard Law School. After Roosevelt appointed him to the Court in 1939, the letters became more telegraphic. They comprise only one segment of the correspondence presented here between Frankfurter and a wide circle of important New Dealers and their British friends. They will substantially modify future Thirties scholarship and throw welcome light on Justice Frankfurter, an imposing historical figure in his own right.