Background and Purpose— The objective of this study was to describe the occurrence of hemorrhagic transformation (HT) among children with arterial ischemic stroke within 30 days after symptom onset and to describe clinical factors associated with HT. Methods— Sixty-three children aged 1 month to 18 years with arterial ischemic stroke between January 2005 and November 2008 were identified from a single-center prospective pediatric stroke registry. All neuroimaging studies within 30 days of stroke were reviewed by a study neuroradiologist. Hemorrhage was classified according to the European Cooperative Acute Stroke Study-1 definitions. Association of HT with clinical factors, systemic anticoagulation, stroke volume, and outcome was analyzed. Results— HT occurred in 19 of 63 children (30%; 95% CI, 19% to 43%), only 2 (3%) of whom were symptomatic. Hemorrhage classification was hemorrhagic infarction (HI)1 in 14, HI2 in 2, parenchymal hematoma (PH)1 in 2, and PH2 in 1. HT was less common in children with vasculopathy (relative risk, 0.27; 95% CI, 0.07 to 1.06; P =0.04) than in those with other stroke mechanisms. HT was not significantly associated with anticoagulation versus antiplatelet therapy (relative risk, 0.6; 95% CI, 0.2 to 1.5; P =0.26) but was associated with larger infarct volumes ( P =0.0084). In multivariable analysis, worse Pediatric Stroke Outcome Measure scores were associated with infarct volume ≥5% of total supratentorial brain volume (OR, 4.0; 95% CI, 1.1 to 15; P =0.04), and a trend existed toward association of worse Pediatric Stroke Outcome Measure scores with HT (OR, 4.0; 95% CI, 0.9 to 18; P =0.07). Conclusions— HT occurred in 30% of children with arterial ischemic stroke within 30 days. Most hemorrhages were petechial and asymptomatic. Infarct volume was associated with HT and worse outcome.