Objective: Accurate measurements of prevalence of "any" breastfeeding and "exclusive" breastfeeding help assess progress toward public health goals. We compared two commonly used data sources for measuring breastfeeding rates to assess agreement. Methods: The National Immunization Survey (NIS) is used by the Centers for Disease Control and Prevention to measure progress toward national breastfeeding goals and obtains breastfeeding outcomes retrospectively at 19-35 months. The California Newborn Screen (CNS) is a contemporaneous measure of breastfeeding during birth hospitalization and measures progress toward public health goals in California. We compared results for "any breastfeeding" and "exclusive breastfeeding" for California infants in the NIS to those in the CNS using descriptive statistics. Results: Our results show that the two methods produce similar results for "any" breastfeeding at <4 days: 82.7%, 95% confidence interval (79.6%, 85.8%) in the NIS and 86.1% (86.0%, 86.2%) in the CNS. However, the two methods produce very different results for "exclusive" breastfeeding at <4 days: 60.4% (56.6%, 64.1%) in the NIS and 41.6% (41.5%, 41.7%) in the CNS. Rates of "exclusive" breastfeeding varied more for some subgroups; for Hispanics, estimates were 61.1% (56.1%, 66.1%) in the NIS and 29.7% (29.5%, 29.9%) in the CNS. Conclusions: There is good agreement between two disparate methods for assessing "any" breastfeeding rates. However, our findings suggest that the NIS, the CNS, or both are flawed measures of "exclusive" breastfeeding. Validated methods for measuring "exclusive" breastfeeding would allow improved monitoring of breastfeeding prevalence.