One notable Deiro achievement was the creation and direction of the world's largest accordion band of 1,500 players for the First Annual Southern California Music Festival held on June 1st, 1940, at the Los Angeles Coliseum. The Coliseum was (and is) one of the largest arenas on Earth and seats 110,000 people. Guido Deiro's accordion band was (and remains after more than a half century) the largest assemblage of accordionists in history.

Los Angeles Times, May 28, 1940

The advertisement and reviews for this event are kept in the Guido Deiro Archive at the City University of New York, and will be submitted to the Guinness Book of Records, which incorrectly lists the present record at 600 accordions.

The Los Angeles Fiesta Day was a big event which included 6,000 participants: 300 majorettes, a 2,500-piece band, a 1,000-voice male chorus, a 50-piece symphony orchestra and 700 dancers. The only group larger than Guido Deiro's 1500-piece accordion band was the [woodwind, brass and percussion] band of 2,500 players.

Guido Deiro figured prominently in the article, which included his photograph plus four paragraphs of text highlighting his accomplishments as the premier pioneer accordionist in America.