Although Gaddes remained involved with the Santa Fe Opera, he left in 1976 to establish, as co-founder, Opera Theatre of Saint Louis and served as its General Director until 1987. Under his leadership, the company achieved international recognition for the development of young artists, the discovery of new singers given the opportunity to perform important roles, and the presentation of a varied repertory. For example, in 1983, Opera Theatre of Saint Louis became the first American opera company to be presented at the Edinburgh International Festival.