In present study, effects of heat stress due to variations in indoor temperature and relative humidity on egg yield and mortality rates of hens in caged poultry houses were investigated between the months April and August (5 months). The experimental poultry house has automated feeder and waterer and operates at 90% capacity. Each cage has 5 hens and there were a total of 9900 Isa Brown hens in the poultry house before the experiments. Hens were 27th weeks old in the beginnings of the experiment. During the experiments, indoor and outdoor climate parameters such as temperature and relative humidity, daily egg production and mortalities were continuously recorded. Structural characteristics of the poultry house were also determined. Heat and moisture gains/losses, temperature humidity index (THI), egg production rates (EPR) and mortality rates (MR) were calculated. Results revealed significantly increasing and strong relationships between indoor temperature and THI-MR and significantly decreasing relationships between indoor temperature and EPR (P<0.01). Indoor temperature increased from 20.7 degrees C in April to 29.4 degrees C in August, THI values increased from 66.1 to 77.0 during the same period. Therefore, mortality rates increased from 0.36% in April to 1.59% in August. While EPR was 88.7% in April, the value decreased to 79.4% in August. Without sufficient wall and roof insulation, it was found to be impossible to provide an indoor temperature of neither 18 degrees C to keep EPR at high levels nor 21 degrees C to keep MR <=parts per thousand 0.1.