Temperature acclimation, either during the incubation or the early growing period, has been suggested as an effective method to improve thermotolerance of broilers to cooler rearing temperatures. We compared the response of broilers from younger (Y) and older (O) breeder flocks to cooler rearing temperatures after temperature acclimation during incubation and early posthatch. Eggs were exposed to either a control incubation temperature (37.6 degrees C) or a cooler temperature (36.6 degrees C; ITcool) for 6 h (from 1000 to 1600 h) daily from d 10 to 18 of incubation. From d 22 to 42, chicks were reared at a control temperature (RTcont), a cooler temperature (17 +/- 2 degrees C; RTcool), or at a cooler temperature after acclimation of chicks to cooler temperatures by exposing them to 17 +/- 2 degrees C for 6 h on d 5 posthatch. The ITcool conditions reduced mortality and mortality due to ascites of O broilers reared under cooler temperatures from d 22 to 42. Early age acclimation to cooler temperatures did not support thermotolerance of broilers. Regardless of IT, the RTcool broilers from Y gained more weight but had poorer feed conversion than RTcont. Repeated cooler temperatures applied during incubation and early postnatal stage disturbed homeostasis and energy balance of broilers from O breeders.