Because the mechanisms of 5-Fluorouracil (5-FU) cardiotoxicity have not yet been completely identified, prophylactic options are not available. To our knowledge, there are no published data investigating the use of angiotensin converting enzyme (ACE) inhibitors for 5-Fluorouracil-associated cardiotoxicity. In this study, we aimed to evaluate the influence of 5-FU administration on the diameter of the brachial artery and the levels of angiotensin II. The patients were administered bolus 5-FU/leucovorin in the study group. Angiotensin II and troponin T assays, complete blood cell counts, hepatic and renal function tests were analyzed in five consecutive blood samples in the initiation, just after termination, and on 24, 48, and 72 h after termination of the regimen. Pre- and post-treatment angiotensin II and troponin T assays, complete blood cell counts, hepatic and renal function tests were also analyzed in the control group. Brachial arterial diameters were measured and recorded in all patients before and after the treatment. A total of 59 patients were included in this study. Thirty one out of 59 patients (52.5%) were in the 5-FU study group and the remaining 28 patients (47.5%) were in the control group. Basal and post-treatment brachial artery diameters in the 5-FU study group were 0.436 +/- A 0.51 and 0.423 +/- A 0.50 cm, respectively (P = 0.001). The corresponding values in the controls were 0.3954 +/- A 0.50 and 0.3957 +/- A 0.49 cm, basal and post-treatment, respectively (P = 0.979). Angiotensin II levels were not changed significantly at serial measurements (P = 0.496). Moreover, the corresponding measurements were not statistically different in both two groups treated with and without 5-FU (P = 0.372). The pathophysiology of 5-FU-induced cardiac toxicity has not yet been elucidated. In the present study, 5-FU-associated vasoconstriction was not dependent on angiotensin II levels, thus we suggest that the prophylactic administration of ACE inhibitors cannot prevent cardiotoxicity in these patients. The underlying mechanisms of cardiotoxicity related to 5-FU might be multifactorial; nevertheless, further prospective investigation for the toxic effects of fluoropyrimidines on the coronary endothelium and myocardium are needed.