OBJECTIVES: To evaluate the safety of trivalent inactivated influenza vaccine (TIV) in children aged 24 to 59 months and to evaluate the risk of medically attended events (MAEs) in a subcohort of children who had multiple annual doses of TIV over their lifetimes. DESIGN: Self-controlled screening study. SETTING: Seven US managed care organizations from October 1, 2002, to March 31, 2006. PARTICIPANTS: Children aged 24 to 59 months who received at least 1 TIV dose (66 283 children and 91 692 TIV doses). EXPOSURE: Vaccination with TIV. MAIN OUTCOME MEASURES: Medically attended events in inpatient and emergency department settings in one of the following risk windows: 0 to 2, 1 to 14, or 1 to 42 days after vaccination. All MAEs that met the screening criteria of incidence rate ratios (IRRs) exceeding 1.0 and P = .05 or IRRs exceeding 2.0 and P < .20 underwent medical record review. A secondary analysis examined the risk of MAEs in children who had multiple annual lifetime TIV doses. RESULTS: Nine diagnoses met the screening criteria. After medical record review, gastrointestinal tract symptoms (IRR, 1.18; 95% confidence interval [CI], 1.10-1.25), gastrointestinal tract disorders (7.70; 1.11-53.52), and fever (1.71; 1.64-1.80) remained significantly associated with vaccination. None of the events seemed to be serious, and none had complications. In the secondary analysis, there was an apparent dose response for vaccine and allergic reactions in the 1- to 3-day risk window. CONCLUSIONS: There was no evidence of serious MAEs following vaccination with TIV among children aged 24 to 59 months. Further studies are warranted to evaluate the risk of MAEs in children with multiple lifetime TIV doses.