Background: Epidemiologic studies have suggested that a higher intake of flavonoids may be associated with lower risk of ischemic heart disease. However, the traditional estimation of flavonoid intake by using dietary assessment methods is affected by subjective measures. Objective: We examined whether the objective measurement of dietary flavonoids excreted in urine is associated with lower risk of acute coronary syndrome (ACS). Design: A case-control study was nested in the Danish Diet, Cancer and Health cohort study. Cases were identified in participants who had received a first-time ACS diagnosis in the Danish National Patient Registry after the time of enrollment into the Diet, Cancer and Health study. The excretion of 10 flavonoids, which represent 5 subclasses, was measured in spot urine samples by using liquid chromatography–mass spectrometry. Results: A total of 393 eligible cases with ACS were identified and matched to 393 noncases by using incidence density sampling. For kaempferol, most of the individual ORs were statistically significant and from 42% to 61% lower when the higher 4 quintiles were compared with the lowest quintile. The P-trend was not significant. For daidzein, individual ORs were 5–38% lower. None of the individual ORs were significant, but the P-trend was 0.041. For the remaining flavonoids, there were no significant relations between urinary excretion and risk of ACS. Conclusions: Except for kaempferol and daidzein, there were no significant associations between the urinary excretion of flavonoids and risk of ACS. A lack of relations may be a result of the use of short-term exposure measures.