Seed quality of 6 sweet narrow-leafed lupin (Lupinus angustifolius) cultivars was measured in 126 field trials in Western Australia over 11 years at 55 locations to determine the effect of locations (l), years (y), genotypes (g), and genotype x environment interactions on total seed alkaloids, seed oil, seed protein, seed size, and hectolitre weight. The variance component for g exceeded the sum of those for g x l, g x y, and g x l x y for all traits. The ranking of cultivars for each seed quality trait was fairly constant across years and locations. The largest variance component was l x y, which indicated that the ranking of locations for seed quality traits was unpredictable from year to year. There was a strong negative correlation between seed oil and seed protein across the 6 cultivars (r = -0.96, P < 0.01) and 126 experiments (r = -0.522, P < 0.001). Large seeds, produced at some experimental sites, were associated with high seed alkaloids and high seed oil. Seed quality traits were not associated with soil pH, latitude, or longitude of the experiments, but low pre-season rainfall was associated with higher hectolitre weight. This study indicates that it should be possible to identify genotypes with superior quality from relatively few field trials. Progress in breeding for protein and oil combined will be slowed by the strong negative genotypic correlation between the two traits.