A group of letters spanning Welch's career, chiefly written to him, but including one early 1887 letter returning a revised manuscript to Dr. Canfield. One notable letter introducing Welch, then at Johns Hopdkins, to Congressman Robert Bremner, is signed by Woodrow Wilson from the White House in 1913. Includes many pieces of correspondence to and from Wilburt C. Davison of the Duke University School of Medicine, including a 1933 telegram to Welch on the occasion of the 2nd anniversary.

ALS. This body of correspondence, almost exclusively letters to Forwood and written immediately before and during the early part of the Civil War, relates to questions of race, e.g. "the Negro problem", intermarriage and consanguinity. The mechanics of editing and publishing a medical journal also form a topic of discussion. Principal correspondents are Samuel Worcester Butler and Washington Chew Van Bibber. Other correspondents are Sylvester David Willard, John H. Van Evrie, J.P. Evans, Joseph Leidy, S.M. Bemiss, James A. Bayard, and Samuel A. Cartwright.