A new disease in broiler breeders known in Australia as Big Liver and Spleen Disease (BLS) is described from field observations, retrospective record analysis, and detailed study of a selected flock. BLS has a predilection for adult birds. It is characterized clinically by a sudden drop in egg production, splenomegaly, hepatomegaly, and increased mortality and histologically by a period of lymphoproliferation followed by a period of lymphoid destruction that coincides with the clinical signs. Epidemiology suggests an infectious cause, though initial attempts at isolation of a causative agent have been unsuccessful, and it is serologically distinct from the common avian viral and mycoplasma diseases.