Fryderyk Chopin’s Fantasy Impromptu in C♯ minor, Op. fast. 66, WN 46 – solo piano composition. It was composed in 1834 and published posthumously in 1855, despite Chopin’s instructions not to publish any of his unpublished manuscripts. Fantasia-Impromptu is one of Chopin’s most frequently performed and popular works. : 189  The Fantasia – Impromptu was written in 1834, as were the Four Mazurkas (Op. 17) and the Grand Brilliant Waltz in E♭ Major (Op. 18), but unlike these other works, Chopin never published the Fantasia – impromptu. Instead, Julian Fontana published it posthumously, along with other Opp waltzes. 69 and 70.

It is not known why Chopin did not release Fantasia-Impromptu. James Huneker called parts of it “sugary” and “devoid of nobility”. Ernst Oster has conducted a technical examination of the work that hints at similarities between Fantasia-Impromptu and Ludwig van Beethoven’s Moonlight Sonata (Quasi una fantasia), which he cites as the reason for Chopin’s reluctance to publish the work. It is also acknowledged to be reminiscent of the Impromptu in E♭ major, Op. 89, compiled by Ignaz Moscheles and published in 1834, the same year that Chopin wrote his Fantasia-Impromptu.

The mystery may have been solved in 1960 when the pianist Arthur Rubinstein purchased the “Album of the Baroness d’Este”, which was sold at auction in Paris. The album contained a manuscript of the Fantasia-Impromptu by Chopin’s own hand, dated 1835, with the inscription on the title page in French “Composed for the Baroness d’Este by Frédéric Chopin”.

The fact that its authenticity had been “guaranteed by the French authorities” and that it showed “delicate care for detail” and “many improvements in harmony and style” over the previously published version was considered by Rubinstein to be absolute proof that it was a finished work. In his preface to The Rubinstein Edition, published by G. Schirmer, Inc. in 1962, Rubinstein suggests that the words “Composed for” instead of the dedication imply that Chopin received a paid commission for the work, so he actually sold it to the baroness.