Ideal soybean [Glycine max (L.) Merr.] production systems achieve both high seed yield and high concentrations of desired seed quality components. However, the relationships between seed quality and yield of soybean are largely unknown. This study sought to determine the relationships of isoflavone, oil, and protein, with seed yield of soybean across a wide range of yield levels. Field experiments involving soybean response to K fertilizer applications in alternate tillage and soybean row-width treatments were conducted at five locations in Ontario, Canada, from 1998 through 2000. Soybean yield and the concentrations and yields of oil, protein, daidzein, genistein, glycitein, and total isoflavone in seed were determined from a total of 13 trials. Oil concentration in seed decreased 4.2 g kg^-1 with each Mg ha-1 of increased seed yield. The relationship between protein concentration and seed yield was not significant. Concentrations of daidzein, glycitein, genistein, and total isoflavone increased by 249, 11, 164, and 427 mg kg^-1 with each Mg ha^-1 of increased seed yield. Overall, oil and protein concentrations were much less responsive to seed yield increases compared with individual and total isoflavone concentrations. Daidzein was the most variable, and glycitein was the most stable of the isoflavone components. Some of the increase of isoflavones with soybean yield may have resulted from a concomitant increase of seed potassium concentrations with yield. Our results suggest that high soybean seed yield can be accompanied by high concentrations of isoflavones without any substantial declines in oil and protein concentrations.