There was little interest in Bartók's compositions in the U.S. and he had difficulty composing. When his health turned bad with what was eventually diagnosed as leukemia, he no longer could make his living giving concerts and doing research and had to depend on his friends to support him. When Serge Koussevitzky commissioned Bartók for a piece for the Boston Symphony Orchestra, his inspiration returned and he composed the Concerto For Orchestra.
Bartók said that he called it a concerto for orchestra rather than a symphony because he treated the instruments in each section of the orchestra in a virtuosic way. It is cast in 5 movements:
I. Introduzione. Andante non troppo - Allegro vivace - Bartók was fond of writing what he called 'night music' as this movement begins with. It then goes through various fugal treatments of themes and it is cast in sonata form.
II. Giuoco Delle Coppie. Allegretto scherzando - The so-called 'game of pairs' begins with the side drum tapping out a rhythm and then each section of the orchestra has 2 instruments play in different intervals, bassoons are a minor sixth apart, oboes are in minor thirds, clarinets in minor sevenths, flutes in fifths and muted trumpets in major seconds.
III. Elegia. Andante non troppo - Another example of Bartók's 'night music', this movement is based on themes first heard in the first movement.
IV. Intermezzo Interrotto. Allegretto - This movement has a melody that is interrupted by Bartók's parody of the march theme from the 7th Symphony 'Leningrad' of Shostakovich. This symphony had been secreted out of Russia on microfilm and a subsequent competition by conductors as to who would conduct the American premiere. The symphony was played over the radio when Bartók heard it.
V. Finale. Presto - This is written in sonata form, and also has some fugal moments along with snippets of folk songs added for good measure.
The Concerto For Orchestra is Bartók's most popular composition but Bartók didn't live long enough to see how popular it became. It is written in a more traditional style than many of his other works. After writing a Sonata For Solo Violin and the Piano Concerto No. 3, Bartók succumbed to leukemia in 1945.