Members of the Collaborative Immunization Initiatives determined the immunization coverage rates for two groups of children in our clinic: those 7 to 12 months old and those 18 to 23 months old. The Clinic Assessment Software Application from the Centers for Disease Control and Prevention was used. The immunization rates determined by this method appeared to significantly underestimate the vaccination coverage rates in our clinic. A review of available charts included in the original sample was done excluding patients no longer attending our clinic. We found a higher rate of coverage in the same sample and a low rate of missed opportunities for administering immunizations. The major reason for this discrepancy is overly stringent Clinic Assessment Software Application inclusion criteria. Additional factors include failure to take into account the wide range of acceptable ages for administering immunizations and different dosages for different brands of vaccines. Different methods of calculation may cause as much as a 20% difference in immunization rates for the same or similar population groups. Such large differences may lead to vastly different responses and interventions. We believe that a central registry is the most accurate method of determining immunization rates. Until this is widely available and applied, a more accurate measure of a facility’s immunization effectiveness is the number of missed opportunities for administering immunizations.