Abstract Broiler breeder performance in a conventional curtain sidewall house and a flexible (FLEX) house from 27 to 64 weeks was compared in two separate trials. Trial 1 was initiated in June of 1982 and Trial 2 in October of 1983. The conventional house was equipped with thermostatically controlled curtains and a stirring (Casablanca type) fan. The FLEX house was equipped with exhaust fans, insulated lightproof curtains, and an evaporative pad system. This allowed the FLEX house to be operated either as a conventional house or as an enclosed house. The breeders in the FLEX house exhibited numerical improvements in egg production, fertility, hatchability, and feed conversion in Trial 1, but opposite results were obtained in Trial 2. These conflicting results are discussed in terms of house temperature, light intensity, and seasonal time of flock placement.