BACKGROUND: Today, ischemic heart diseases (IHDs) are the most common diseases worldwide. Angiography is the best way to diagnose IHDs. Angiographic complications however can include death, myocardial infarction, nausea, spasm of the coronary arteries, chest pain, bradyarrhythmia, hypotension, or hypertension. This study aimed to determine the effects of a simultaneous injection of trinitroglycerin (TNG) with contrast agent on the early complications of angiography. METHODS: This clinical trial was conducted in Chamran Hospital, Isfahan, Iran in 2005. Before the angiography, the study population were randomly assigned into two groups of intervention (n = 111) and placebo (n = 112). TNG was mixed with contrast material in the intervention group and distilled water was mixed with contrast agent in the placebo group to be used for angiography. Nausea, spasm of the coronary arteries, chest pain, bradyarrhythmia and the mean systolic and diastolic blood pressures were compared during and after the angiography in patients of both groups. The obtained data was analyzed by chi-square and t tests. RESULTS: The mean age in the intervention and placebo groups were 59.93 ± 9.14 and 59.37 ± 10.12 years, respectively (P > 0.05). The two groups were not significantly different in terms of gender distribution. The frequency of nausea was 4.5% and 6.2% in the intervention and placebo groups, respectively. The corresponding values were 0.9% and 5.4% for coronary artery spasm and 1.8% and 6.3% for chest pain. Bradyarrhythmia occurred in 7.1% of patients in both groups. There were no significant differences in the abovementioned complications. Mean values of the highest and lowest systolic and diastolic blood pressures of the intervention group were significantly different from the placebo group (P = 0.001). Simultaneous injection of TNG and the contrast agent in this study resulted in positive findings (except for changes in blood pressure). CONCLUSION: More studies are recommended with different doses of TNG, different times of injection and considering at-risk individuals. Keywords: Angiography, Early Complications, Trinitroglycerin.