Critics of modern maize and wheat varieties (MVs) have suggested that, in developing countries, yields of these varieties vary more from season to season than yields of farmers’ traditional varieties, thereby exposing consumers and producers to greater risk. Drawing on country-level data for MV diffusion, as well as aggregate data on production and yields from FAOSTAT, this study makes novel use of the Hodrick-Prescott filter to disentangle changes in trend from annual fluctuations. The outcomes suggest that, over the past 40 years, the relative variability of grain yields—that is, the absolute magnitude of deviations from the yield trend—has declined for both wheat and maize in developing countries, and that the reduction is statistically associated with the spread of MVs, even after controlling for expanded use of irrigation and other inputs. At appropriate world prices, the annual benefits from improved yield stability alone are about US$143 million for wheat and about US$149 million for maize.27 pages