Purpose. This study aimed to evaluate the accuracy and outcomes of focused assessment with sonography for trauma (FAST) and determine the factors associated with true-positive FAST results. Methods. The FAST results from 2016 to 2020 were retrospectively reviewed. Cases involving penetrating injury, transfer from other hospitals, age ≤ 16 years, prehospital arrest, and no confirmatory test were excluded. Intra-abdominal fluid was confirmed using computed tomography or operative findings. The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated. Demographic data, injury characteristics, and outcomes were compared between true-positive and false-negative results. Logistic regression was used to identify the factors associated with true-positive results. Results. Of 2,758 patients, 163 and 2,595 patients showed positive and negative results, respectively. True positives were 135 and true negatives were 2325. The overall sensitivity, specificity, PPV, and NPV were 33.3%, 98.8%, 82.8%, and 89.6%, respectively. The sensitivity increased to 49.1% in patients with initial systolic blood pressure (SBP) ≤ 90 mmHg. The true-positive group showed a lower SBP and Glasgow Coma Scale score and a higher laparotomy rate than the false-negative group. However, mortality showed no significant difference. In logistic regression analysis, hollow viscus injury (1.820 [1.123–2.949], P = 0.015 ) and the lowest SBP (0.988 [0.980–0.997], P = 0.009 ) were associated with true-positive results compared to false-negative results. Conclusion. The overall sensitivity of FAST was low; therefore, it should be performed in selected patients such as SBP ≤ 90 mmHg. Because of its low sensitivity and no influence on outcome, physicians should not rely solely on FAST.