John Keats died in Rome in February 1821, at the age of 25. Shelley, who was to follow him to the grave the following year, was inspired to compose this pastoral lament, which consists of 55 Spenserian stanzas.

Shelley bitterly, but incorrectly, attributed his friend's untimely passing essentially to Keat's extreme reaction to the savage criticism of his major work, Endymion. In the preface to Adonais, Shelley describes Endymion as "one of the noblest specimens of the workmanship of God," and in the poem, he states that: "till the Future dares forget the Past, his fate and fame shall be an echo and a light unto eternity!"