The main arena of the Cleveland Public Auditorium is one of the largest listening chambers in the world. Built in the early 1920's, it has, since its opening, been the scene of important public functions, including presidential conventions. Criticism of its acoustic qualities had existed from its opening, but had increased rapidly since 1950, and in 1955 the city administration called for a thorough study of its acoustic qualities and recommendations for improvement. This paper consists of a description of the initial acoustic study, including articulation testing and reverberation time measurements, the problems involved in the acoustic treatment, the actual application of the treatment, and opinions of users since completion.