Isoflavones in soybean [Glycine max (L.) Merr.] may have positive impacts on human health. The objective of this study was to determine the role of the genotype, environment, and genotype x environment interactions on isoflavone contents of soybean. Nine isoflavones were measured in six cultivars grown at eight locations during 2 yr. The mean contents of total isoflavones and six of nine individual isoflavones were significantly higher in 1996 than 1995. There were significant differences among locations in one or more years for total and individual isoflavone contents. The year x location interactions were significant due to changes in rank and magnitude among the locations during the 2 yr. The genotype, genotype x year, genotype x location, and genotype x year x location interactions were significant for total and individual isoflavone contents. Despite the significant genotype x environment interactions, the differences between the cultivars with the highest and lowest total and individual isoflavone contents were relatively consistent among the 16 environments. It should be possible to breed for isoflavone content as a quantitative trait in a cultivar development program.