Since 1998, the Florida Department of Health (FDOH) has operated the Florida Birth Defects Registry (FBDR), a statewide, population-based, passive surveillance system. Cases are identified by collecting information from extant data sources including the statewide hospital inpatient and ambulatory discharge data sets. Additional data sources include administrative, clinical, and service-related information from the FDOH's Children's Medical Services program for children with special health care needs. Like many state birth defects programs, the FBDR faces diminishing funding and resources that may restrict the registry to hospital discharge data. We conducted an evaluation to quantify the potential under-ascertainment to the FBDR resulting from loss of specific data sources, and to determine if there would be a disproportionate loss of cases by sociodemographic and perinatal characteristics. Analyses involved a series of retrospective reconstructions of the FBDR for 1998-2007 to assess the number of cases that would have been ascertained and reported based on the hypothetical loss of 1 or more of the FBDR source data sets. The reconstructed number of cases identified for each defect category was then compared to the current FBDR (constructed using all 5 source data sets) to determine the proportion of cases that would have been missed if the data sources in question were eliminated. These scenarios were constructed overall and by selected characteristics to identify potential disparities in the proportion of cases missed. The inpatient hospital discharge data set was the primary data source for identification of birth defects in the FBDR. Elimination of this single data source would cause the FBDR to miss nearly three fourths of infants diagnosed with 1 or more of the birth defects under study. Our evaluation revealed that an FBDR constructed on hospital discharge data alone would disproportionately miss more cases born to subgroups of women, including non-Hispanic blacks, Hispanics, and those born outside the US. Despite funding and resource constraints, the FBDR continues efforts to identify data sources that may contribute to completeness of case ascertainment in an effort to serve the needs of the Florida maternal and child health population.