Although rare, numerous case reports suggest that Thebesian veins confer increased morbidity and mortality. No study has evaluated their effects on cardiac structure or long-term patient outcomes. Patients undergoing coronary angiogram at the study institution from October 2002 and January 2015 were assessed for a diagnosis of prominent Thebesian veins. A matched control group was created and comparisons between clinical, echocardiographic, and survival measures were made. Of 50,116 total patients, 31 (0.06%) were found to have prominent Thebesian veins on angiography and were compared with a matched control group of 596 patients. Patients were matched for age, gender, angiogram date, location and extent of coronary disease, and dominance. Demographic and clinical data were similar between cohorts, with a median follow-up period of 26 months. Patients with Thebesian veins had lower Doppler E wave (0.7 vs 0.8; p = 0.007) and A wave (0.6 vs 0.8; p = 0.001) mitral inflow velocities suggesting some decrease in normal mitral inflow, potentially due to direct shunting into the ventricle from the Thebesian vein network. However, there was no observed difference in left ventricular size or ejection fraction between groups. There was also no significantly increased mortality associated with the presence of Thebesian veins (hazard ratio 1.11, 95% CI 0.58 to 2.13). In conclusion, although previous reports have suggested adverse outcomes from Thebesian veins, our case–control study demonstrated no significant associated adverse cardiac structural changes or increase in mortality, although patients with Thebesian veins were noted to have a decrease in mitral inflow velocities.