About 16% of children have developmental-behavioral disabilities but less than one-third of the children are detected by their health care providers, probably because of the use of informal milestones checklists. The goal of this study is to determine the reliability, validity, accuracy, and utility of a new tool, PEDS®: Developmental Milestones (PEDS:DM®). Data from a nationally representative sample of 1619 children administered developmental diagnostic measures were mined for items that best predicted performance in each developmental domain. A total of 112 met inclusion criteria, that is, sensitivity/specificity ≥70%. For each domain/age level (birth to 8 years of age), ensitivity to performance less than or equal to the 16th percentile on diagnostic measures was 83% and specificity was 84%. Reliability was high (test–retest, .98 to .99; interrater, .82 to .96; κ, .81). The readability level was 1.8 grades (range 1.1 to 2.6). The PEDS:DM® appears to be a validated, accurate alternative to informal milestones checklists that are a probable contributor to underdetection of children with delays and disabilities.