Background: An accurate measurement of patient weight is important in determining the dosage for intravenous alteplase thrombolysis. In most emergency rooms, however, weight is not measured. We investigated the difference between stated and measured weight and its effect on hemorrhagic transformation and clinical outcomes. Methods: We enrolled 128 consecutive patients who had hyperacute stroke and were treated by alteplase. Alteplase dose was calculated using the weight provided by patient or guardian/caregiver, and the actual weight was measured after administration. Patients were classified into 2 groups: overused group (stated weight >measured weight) and underused group (measured weight ≥stated weight). The prevalence of hemorrhagic transformation on follow-up, determined by gradient-recalled echo MRI or non-enhanced CT, was compared between the 2 groups. The predictors for hemorrhage with progression, defined as an increase in the National Institutes of Health Stroke Scale (NIHSS) by a value of 4 or more accompanied by hemorrhage, were determined using multivariable logistic regression analysis and included the overused or underused alteplase and baseline clinical and laboratory findings. Results: Sixty-six (51.6%) of 128 patients were in the underused group and 62 patients (48.4%) in the overused group. The median difference between the stated and measured weights was 1.5 (interquartile range 0.56-3.81) kg, with the largest difference being 25.6 kg. Although there were no significant difference in baseline clinical and laboratory findings between the 2 groups, the overused group showed a significantly higher prevalence of hemorrhagic transformation (p = 0.012) and hemorrhage with progression (p = 0.025). The multivariable logistic regression analysis demonstrated that overused alteplase (OR 7.26; 95% CI 1.24-42.45; p = 0.028), baseline glucose (>144 mg/dL; OR 5.03; 95% CI 1.00-25.26; p = 0.050), and initial NIHSS (OR 1.13 per 1-point NIHSS increase; 95% CI 1.00-1.27; p = 0.047) in model 1 that use alteplase overdose as a categorical variable and overused alteplase (OR 1.67 1-mg increase; 95% CI 1.05-2.66; p = 0.027) in model 2 that use an overused alteplase dose as numerical variable were significant predictors for hemorrhage with progression. Conclusion: More alteplase usage than actual weight led to higher hemorrhagic transformation. As one of the predictors for clinical deterioration, it is important to administrate alteplase based on an accurately measured weight.