This is one of the most important of Lutoslawski's compositions in terms of spreading his reputation worldwide. It was written for performance in Venice in April 1961. Only three movements of the four (the third was as yet unready) were premiered there and the whole piece was first played at the Warsaw Autumn Festival of the same year.

In this piece Lutoslawski expands on an interest in aleatoric music. This term has been coined from the Greek word for "dice" meaning that chance elements are present in the music. As Lutoslawski applied the idea, he frequently leaves to performers some of the choices that are normally dictated by the composer. These are usually in the realm of time, so that strands of music might match up differently from performance to performance. Sometimes the players can choose notes from a limited selection, resulting in the desired harmony but difference in melodic details.

The opening movement is divided into eight sections labeled from A to H. Each begins with a signal from percussion to tell the players when to move from one section to another. Each section corresponds with a box written in the score showing what can be played within the section. The sections contrast with each other like a series of refrains and episode. Instruments are generally restricted to playing certain intervals within gigantic aggregates of two 12-note chords.

The second movement is through-composed, a scherzo in triple meter that simply serves as a transition. In the third movement, the piano plays a twelve-tone row melodically, made of only perfect fourths and perfect fifths, in its prime and altered forms. Meanwhile, the other instruments join in a succession of 16 chords. The final movement is even more aleatoric than the opening one, with overlapping and contrasted instrumental blocks. It is the overlapping and piling on of these blocks that build it to a powerful climax. Once again, 12-note chords dominate the harmony. This is obviously hard and difficult music for most listeners. Even for experienced listeners, these blocks merge into a single sound of dense, shifting textures.

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