PURPOSE: To evaluate outcomes of botulinum toxin (BTX) injection of the inferior oblique (IO) muscle. DESIGN: Retrospective case series. METHODS: Setting: Single center, ophthalmology department at Boston Children's Hospital. STUDY POPULATION: All patients treated with IO muscle injection of BTX (onabotulinumtoxinA) between 2010 and 2020. OBSERVATION PROCEDURE: Sensorimotor evaluations at short-term (<2 months), medium-term (2-4 months), and long-term (≥4 months) intervals. OUTCOME MEASURE: Primary outcomes included median improvement in V-pattern strabismus and primary position hypertropia. Secondary outcomes included IO muscle overaction. Wilcoxon signed-rank tests were performed to identify differences before and after injection. RESULTS: Record review identified 20 patients with a median age of 4.5 (range, 1-69) years. Median BTX dose injected (31 IO muscles) was 5.0 (range, 3.0-7.0) units. Indications included V-pattern strabismus (N = 8), hypertropia (N = 7), or both (N = 5). Median long-term interval was 6.4 (range, 4.1-26.6) months. Injections were concurrent with treatment of horizontal strabismus in all but 3 cases. Median V-pattern magnitude changed from 10 prism diopters (PD) preoperatively to 0 PD short-term (P = .006) and 3.5 PD long-term (P = .34). Median hypertropia changed from 8.5 PD preoperatively to 1.5 PD short-term (P = .01) and 8 PD long-term (P = .87). Median IO muscle overaction grade improved significantly at short-term (P < .001) and long-term (P = .007) intervals. There were no complications associated with the IO muscle injections. CONCLUSIONS: BTX injection of the IO muscles can be a useful adjunct to the management of V-pattern strabismus. Intervention for primary position hypertropia may be helpful for short-term relief with no expectation of long-term benefit.