Background: To evaluate the relationship between body weight and the success rate of supraventricular tachycardia (SVT) termination after an initial 6-mg adenosine administration. Methods: This retrospective observational study was conducted with medical records of patients who visited a single tertiary emergency department and underwent an electrophysiologic study with an SVT diagnosis. Patients under 18 years of age or those with SVT aberrancy, recurring SVT after radiofrequency catheter ablation, premedication of non-dihydropyridine calcium channel blocker, and signs of shock were excluded. Results: Data from 124 patients were collected. There were 69 women (55.6%); mean age, 49.7 years. The mean weight and height were 66.8 kg and 164.8 cm, respectively. Adenosine dose was 0.09 mg/kg. For the first sinus conversion (SC) success group (83 patients, 67%), the mean weight, height, and adenosine dose were 61.0 ± 11.5 kg, 161.9 ± 8.1 cm, and 0.10 ± 0.02 mg/kg. For the first SC failure group, these values were 78.4 ± 17.0 kg, 170.7 ± 8.9 cm, and 0.079 ± 0.02 mg/kg, respectively. To evaluate factors associated with the first SC, we performed bivariate logistic regression with weight (odds ratio [OR]: 1.093; 95% confidence interval [CI]: 1.045–1.144; P < 0.001) and height (OR: 1.071; 95% CI: 1.008–1.138; P = 0.027). Receiver operating characteristics curves and the cut-off values for adenosine dose and weight were 0.90 mg/kg and 66.5 kg with 71.1% sensitivity and 73.2% specificity (area under the curve: 0.814; 95% CI: 0.73–0.90; P < 0.001). Conclusion: Weight and height were significant predictors of the first SC success rate among SVT patients. Consequentially, it is necessary to consider other adenosine administration methods, including increasing the initial drug dose or changing the flushing strategy after adenosine injection.