Background: Numerous factors influence young children's readiness to learn when they enter kindergarten. An index that quantifies the risk of inadequate preparation for school among children across Oklahoma could help the state track the prevalence of risk factors and target programming to areas most in need. This paper describes the development of the Oklahoma School Readiness Risk Index (SRRI) 2013, which updates the SRRI originally developed in 2011. Methods: Based on a review of the empirical literature that identifies factors related to school readiness, and on availability of data at the county level, 18 variables were selected for analysis and organized into Infant/Child, Maternal, and Family related factors. Data for these variables were compared to national-level data. Principal Components Analysis (PCA) was used as a dimension reduction technique to analyze the 18 variables, followed by multivariate regression methods to identify factors that significantly explain school readiness, using third-grade reading and mathematics proficiency as outcome variables. Results: The PCA yielded four components comprised of 12 variables, with 6 variables that did not correlate with any component. Regression analysis showed that, when combined, variables on 3 components (low maternal education, Hispanic ethnicity, English language learners, young maternal age, single-parent family, family poverty status, American Indian race, abuse and neglect, and children in foster care) and 1 individual variable (child migrant status), significantly explained third-grade reading skills but were not significantly associated with third-grade mathematics skills. These variables were then used to calculate scores on the SRRI for each county, which show that 58 out of 77 Oklahoma counties have, on average, higher risk indices than national rates. In other words, approximately 79% of all Oklahoma children from infancy to age 5 are at high risk of coming to school unprepared to learn. Conclusion: Development of the Oklahoma SRRI 2013 is an important step in monitoring the severity of risks for poor school readiness faced by Oklahoma's children, and highlights the extreme need for early childhood education programs across the state. (author abstract)