OBJECTIVES. Children <9 years of age do not respond optimally to a first dose of trivalent inactivated influenza vaccine, and so 2 doses of trivalent inactivated influenza vaccine are recommended for children <9 years of age who are being vaccinated for the first time. We conducted a population-based retrospective cohort study to evaluate compliance with the 2-dose trivalent inactivated influenza vaccine recommendations. POPULATION AND SETTING. We evaluated 125928 children 6 months through 8 years of age who were enrolled in health maintenance organizations in the United States participating in the Vaccine Safety Datalink project and who received their first dose of trivalent inactivated influenza vaccine in the 2001–2002, 2002–2003, or 2003–2004 influenza seasons. RESULTS. Compliance with the 2 dose recommendations varied by age group and influenza season. Among children 6 to 23 months of age, the proportion of first-vaccinated children who received a second vaccination was 44% in 2001–2002, 54% in 2002–2003, and 29% in 2003–2004. Among children 2 to 8 years of age, the corresponding proportions were 15%, 24%, and 12%, respectively. In all seasons, compliance with the second vaccination was highest in children first vaccinated by mid-November. CONCLUSIONS. The majority of children who received their first dose of trivalent inactivated influenza vaccine did not complete the 2-dose series. The recently expanded recommendation for universal vaccination of children 6 to 59 months of age and their household contacts will substantially increase the number of children targeted for a first influenza vaccination. Noncompliance with the 2-dose trivalent inactivated influenza vaccine series may be associated with suboptimal protection against infection, which may impact the magnitude of the direct and indirect benefits achieved by the vaccination program.