Agaricus brunnescens Peck sometimes is synonymized with A. bisporus (J. E. Lange) Imbach, the cultivated button mushroom. However this relationship has not been accepted universally or unreservedly. For various reasons A. bisporus is the more commonly used name. This study demonstrated that A. brunnescens is based on a heterogeneous type, as suggested by elements of the type description, correspondence at NYS and a lack of consensus among studies of the type. DNA sequence analysis indicated that material corresponding to both A. bisporus and A. subrufescens Peck is present in the type as presently constituted. A lectotype corresponding to the concept of A. bisporus is chosen, and nomenclatural issues are discussed. A definitive history of A. brunnescens is attempted.