over a 15-yr period (1997-2011), the relationship between growing-season temperatures and both sink components and yield of two elite winter wheat (Triticum aestivum L.) cultivars, Hokushin (soft red) and Kitanokaori (hard red), was investigated in Hokkaido, Japan. Culm length, spike length, duration from heading to maturation, and thousand-grain weights showed a significant inverse correlation to mean seasonal ambient temperatures. In particular, higher temperatures during vegetative stage significantly reduced the magnitude of sink components. Specifically, a 1°C rise in temperature in late May resulted in spike length being shortened by 2.8 mm (3.2%) for Hokushin and 4.4 mm (4.5%) for Kitanokaori. Higher temperatures from heading to maturity significantly shortened the duration from heading to maturation and resulted in a smaller grain size. Specifically, a mean rise in temperature of 1°C from heading to maturation resulted in a 2.8-d (6.4%) shortening of the duration from heading to maturation in both cultivars, and a concurrent decline in thousand-grain weight of 2.3 g (6.1%) for Hokushin and 1.9 g (4.6%) for Kitanokaori. Given the dissonance between the large sink size and limited supply of assimilates arising from lower spring and higher summer temperatures, severe yield reductions can be expected. The maintenance of higher grain weight under heat stress, such as exhibited by Kitanokaori, can serve as a useful measure of heat tolerance in wheat. © Crop Science Society of America.