Purpose: To determine whether specific ultrasonographic features can predict failure of conservative treatment of acute appendicitis. Methods: A 2-year retrospective study was conducted on children admitted with acute appendicitis. Those with uncomplicated appendicitis diagnosed solely by ultrasound, and treated conservatively, were followed 18–24 m to assess treatment outcome. Management was considered successful if recurrent acute appendicitis was not observed during follow-up. Appendix diameter, wall thickness, presence of mucosal ulceration, hyperechogenic fat, free fluid, and lymph nodes were evaluated as potential discriminatory ultrasonographic predictors. T-tests, chi-square, sensitivity, specificity, and odds ratios were calculated. Results: Out of 556 consecutive patients that were admitted with acute appendicitis, 180 (32%) managed conservatively. One hundred eleven (62%) imaged by US only. Ninety-two out of 111 (83%) were followed 18–24 m to assess treatment outcome, and 19/111 (17%) were lost to follow-up. Conservative management was successful in 72/92 (78.2%), with treatment failure in 20/92 (21.8%) (5/92 (5.4%) with recurrent acute appendicitis and 15/92 (16.3%) underwent appendectomy). Of the ultrasonographic features studied, mucosal ulceration demonstrated statistically significant predictive value. Fifteen out of 20 (75%) treatment failures had mucosal ulceration, compared to 21/72 (29.2%) of the patients with successful treatment (p < 0.001). This yielded a positive odds ratio of 7.3 (2.3–22.6, 95% CI), 70.8% (58.9–80.9, 95% CI) specificity, and 75% (50.9–91.3, 95% CI) sensitivity. Positive predictive value was 41.6% (31.5–52.5, 95% CI) while intact mucosa had negative predictive value of 91% (82.4–95.6, 95% CI) for conservative management success. Conclusion: The presence or absence of appendiceal mucosal ulceration at ultrasound can predict conservative management outcome in the setting of acute appendicitis, potentially improving pediatric patient selection for conservative management.