OCT requires intracoronary injection of contrast agent to remove blood from the coronary lumen during data acquisition, which is a possible limitation of this method. Aim of this study was to analyze the influence of iodine concentration on image quality and diagnostic certainty of optical coherence tomography (OCT). OCT sequences acquired using contrast agent with a reduced concentration of 150 mg iodine/ml and a standard concentration of 350 mg iodine/ml were analyzed. Cross-sectional images with a spacing of 10 mm were evaluated regarding image quality and diagnostic confidence. A total of 67 OCT sequences acquired in 24 patients were analyzed. 31 sequences were acquired using contrast agent with a concentration of 150 mg iodine/ml and 36 sequences with a concentration of 350 mg iodine/ml. The percentage of remaining blood streaks in the cross sections was significantly lower for 350 mg iodine/ml compared to 150 mg iodine/ml (19 ± 21 vs. 34 ± 26%, p = 0.013). Contrast with 350 mg iodine/ml showed a significantly higher percentage of completely flushed pullback length as compared to 150 mg iodine/ml (78 ± 24 vs. 58 ± 27%, p = 0.004). Diagnostic certainty was significantly higher for 350 mg iodine/ml than for 150 mg iodine/ml (Likert scale average 1.4 ± 0.7 vs. 2.1 ± 1.2, p < 0.001; Likert scale: 1 = absolutely confident, 2 = confident with slight doubts, 3 = doubtful/not confident, 4 = non-diagnostic). Regarding image quality and diagnostic certainty, contrast agent with a concentration of 350 mg iodine/ml is superior to 150 mg iodine/ml.