Duruflé's greatest composition, the Requiem, Op. 9, completed in September 1947, enjoys a reputation as one of the undisputed masterpieces of the twentiethcentury choral repertoire. The single piece most responsible for establishing his fame worldwide, it continues to enjoy frequent performances in the West and the East alike. Reviewers have described it as softly luminous, sumptuous, suffused with a tender radiance, of a noble and restrained eloquence and a sweet and serene light, a work of scrupulous craft and exquisite sensibility, having beautiful unity and real grandeur.

For a long time Duruflé had been seduced by the beauty of the Gregorian chants from the Mass for the Dead. He said:

Composition

Duruflé composed the piece during the day on the Elke upright piano at his mother's home in Louviers, and in the evening would refine his work on the organ in the church.

Duruflé has been credited as the first to compose a requiem based upon the chants of the Gregorian Missa pro defunctis, but he was, in fact, not the first. In his nine-movement Missa pro defunctis for six voices (SSATTB), the Renaissance composer Tomas Luis da Vittoria incorporated the Gregorian cantus verbatim, assigning it to the second soprano part throughout. Vittoria's setting lacks parts that Duruflé included, namely, the Pie Jesu and the In paradisum. But insofar as it lacks the Dies irae, Vittoria's setting is similar to Duruflé's. Whether Duruflé knew of this work is a matter of conjecture. In any case, Duruflé's is the first such setting since the Renaissance.