Two mild and two pungent onion (Allium cepa L.) selections (hereafter referred to as cultitypes), W420B, W424B, MSU8155B, and Exhibition, were grown at two locations in two states (Wisconsin and Oregon) during 1994 and 1995. Onion bulbs were harvested, stored at 4 °C and sampled for antiplatelet activity, pungency, and soluble solids 10 days after harvest and every 40 days during a 210-day postharvest storage period. Significant cultitype × state and cultitype × year interactions were detected. However, these were primarily due to the change in rank of cultitypes within the mild or pungent group. Averaged over all environments, antiplatelet activity was significantly greater in 1994 compared to 1995 for all cultitypes. Significantly greater antiplatelet activity was measured for three out of four cultitypes grown in Oregon compared to Wisconsin. During postharvest storage, antiplatelet activity increased 61% and 56% across all cultitypes and across both states during 1994, and across all cultitypes in Wisconsin during 1995, respectively. Although pungency determination can be a good indicator for relative rankings of different cultitypes for antiplatelet activity, changes in pungency were not correlated with changes in antiplatelet activity during postharvest storage. Results demonstrate cultitype, environment, duration of postharvest storage and genotype × environment interactions influence pungency, soluble solids, and antiplatelet activity, which should be considered when assessing onion-induced antiplatelet activity.