A series of studies were performed to determine the effects of increasing environmental temperature and relative humidity on the performance of Holstein heifers. In heifers of 200 kg body weight, the effects on dry matter intake and daily weight gain were small at 28°C and 60% relative humidity, but dry matter intake and daily weight gain were seen to start declining at 28°C and 80% relative humidity. Moreover, in heifers of 400 kg body weight, even at relative humidity of 60%, dry matter intake and daily weight gain started to decline at 28°C. At 60% relative humidity, the rate of decline in daily weight gain in heifers of 200 kg body weight at 33°C was similar to that of those of 400 kg body weight at 28°C. These results indicated that the influence of high temperature on the performance of heifers intensified at a late stage, and under high relative humidity. An assessment of the impact of global warming on heifer growth performance in the Japanese summer was made using 'Climate change 10 × 10 km mesh data (Japan)', which is one of the models assuming progressive global warming. In the 2060s, compared to the end of the 20^th century, areas showing a decline in the rate of body weight gain in heifers will expand from Kanto to the western Hokkaido area, and the rate of decline will accelerate over time from the present to the 2020s, 2040s and 2060s, especially in July and August. As high humidity under hot temperatures significantly affect heifer growth, global warming and high humidity in the Japanese summer will have the negative effect.