Harvest index (HI; the ratio of grain yield to above-ground biomass at maturity) has been proposed as a selection criterion for use in plant breeding programmes. There is as yet no consensus, however, on the value of using the trait. This work was undertaken to provide some information necessary for determining the value of the trait. It was conducted with both winter and spring wheat (Triticum aestivum L.) genotypes at six locations in Ontario, Canada in the 1993/94 and 1994/95 cropping seasons. Stability analysis revealed that in winter white wheats, grain yield was more stable than HI, whereas the inverse situation held for spring wheats. Analysis of dry weight at anthesis relative to at maturity suggested that pre-anthesis stem reserves could be one of the factors responsible for differences in the stabilities of HI and grain yield. Correlation analyses of HI and its components and sub-components showed that HI was positively and highly correlated to kernel weight in spaced planting and to spike to stem ratio at anthesis in dense plantings. Such findings indicated that HI in spaced planting was determined after anthesis, whereas in dense planting it was determined before anthesis. Most shoot traits were correlated to grain yield per shoot but not to grain yield per unit area; spike to stem ratio at anthesis and kernel number/shoot biomass at anthesis were negatively correlated with grain yield per unit area. Two-way selection for high and low HI showed that HI was weakly heritable (h\sp2 = 27% to 55%). Overall, the results indicated that both HI and grain yield can be significantly affected by environmental changes, and are unstable, but in different ways. HI was not correlated with grain yield in dense planting and was also of low heritability. Together, the findings indicate that HI is not an appropriate indicator of grain yield in a crop situation. Negative correlations between some shoot traits and grain yield per unit area indicate that attention should be placed on aspects that determine spike number per unit area in a crop situation.