Cyril Allinson was a sergeant major in McCrae's unit. While delivering the brigade's mail, he watched McCrae as he worked on the poem, noting that McCrae's eyes periodically returned to Helmer's grave as he wrote. When handed the notepad, Allinson read the poem and was so moved he immediately committed it to memory. He described it as being "almost an exact description of the scene in front of us both". According to legend, McCrae was not satisfied with his work. It is said he crumpled the paper and threw it away. It was retrieved by a fellow member of his unit, either Edward Morrison or J. M. Elder, or Allinson himself. McCrae was convinced to submit the poem for publication.
Another story of the poem's origin claimed that Helmer's funeral was actually held on the morning of May 2, after which McCrae wrote the poem in 20 minutes. A third claim, by Morrison, was that McCrae worked on the poem as time allowed between arrivals of wounded soldiers in need of medical attention. Regardless of its true origin, McCrae worked on the poem for months before considering it ready for publication. He submitted it to The Spectator in London, but it was rejected. It was then sent to Punch, where it was published on December 8, 1915. It was published anonymously, but Punch attributed the poem to McCrae in its year-end index.
The word that ends the first line of the poem has been disputed. According to Allinson, the poem began with "In Flanders Fields the poppies grow" when first written. However, since McCrae ended the second-to-last line with "grow", Punch received permission to change the wording of the opening line to end with "blow". McCrae himself used either word when making handwritten copies for friends and family. Questions over how the first line should end have endured since publication. Most recently, the Royal Canadian Mint was inundated with queries and complaints from those who believed the first line should end with "grow" when a design for the ten-dollar bill was released in 2001 that featured the first stanza of "In Flanders Fields", ending the first line with "blow".