Background: Fixed dose contrast medium (CM) injection protocols are often used in CT coronary arteriography (CTCA). However, vascular enhancement is related to body weight and injection dose rate. Purpose: To compare aortic enhancement and coronary imaging quality between a fixed dose CM injection protocol and one adapted to body weight with a constant injection dose rate in mg l/kg/s. Material and Methods: One hundred consecutive patients undergoing CTCA were randomized to two groups; group I received 65 mL CM (370 mg l/mL) at 5 mL/s and group II received 0.9 mL CM (370 mg l/mL) per kg during 12 s resulting in a constant injection dose rate of 28 mg l/kg/s. The groups were re-divided according to body weight: IA <= 70 kg, IB > 70 kg, IIA <= 70 kg, IIB > 70 kg. Aortic attenuation was measured and coronary imaging quality was graded using a four-grade scale. Results: The resulting mean CM dose, aortic attenuation and score of coronary imaging quality in groups IA/IB/IIA/IIB were 24/24/19/27 g 1, 437/403/448/440 HU and 2.8/2.3/2.7/2.6, respectively. CM dose and aortic attenuation were significantly different between groups IA and IIA (p < 0.001, p = 0.01), as well as between groups IB and IIB (p < 0.001, p < 0.001). There was no statistically significant difference regarding coronary imaging quality between groups IA and IIA (p = 0.30), while there was a significant difference between groups IB and IIB (p < 0.001). Conclusion: In CTCA, body weight-adapted CM injection protocols using a constant dose rate can help save CM doses in slimmer patients and improve aortic enhancement and coronary imaging quality in heavier patients compared to fixed dose protocols.