Question: What well-child visit data in the first 5 years of life are significantly associated with subsequent kindergarten readiness? Findings: In this cohort study, 3204 primary care patients were matched with school district Kindergarten Readiness Assessment (KRA) scores. After adjusting for covariates, lower KRA scores were significantly associated with the following variables: child ever failed developmental screening after 18 months, child was ever rarely read to, family reported food insecurity, Medicaid insured, Hispanic ethnicity, family required an interpreter, late COVID-19 pandemic school year, and child male sex. Meaning: Results suggest that there is an identifiable population of children who may benefit substantially from early systematic collaboration between clinicians and educators. (author abstract)