The thermal environment in poultry housing is a primary influence on production efficiency and live performance. Heavy broilers (body weight > 3.2 kg) typically require high ventilation rates to maintain thermal comfort and production efficiency. However, large birds are observed to pant in mild to moderate thermal conditions, indicating that upper critical temperatures may be lower at larger body weights. Thermal comfort indices such as the temperature-humidity index (THI) integrate the effects of temperature and humidity and may offer a means to predict the effects of thermal conditions on performance. The objective of this study was to determine live performance of heavy broilers over a range of dry-bulb temperature (15°C, 21°C, and 27°C) and relative humidity (50%, 65%, and 80%), hence THI (14.8°C to 26.9°C). A series of four studies were completed with broiler chickens housed in environmental chambers. Live performance parameters including body weight, body weight gain, feed intake, and feed conversion ratio were compared; body temperature was measured in three birds of each treatment during one study. Results show that as THI exceeds approximately 21°C, bird performance significantly declined and body temperature increased up to 1.7°C above nominal body temperature for broilers (41°C). Regression analysis showed that a quadratic relationship exists between THI and the four performance parameters of interest. Prediction accuracy was decreased due to variability in the data and suggests data at additional THI points are necessary.