This project sought to identify Medicaid members with intellectual and developmental disabilities (IDD) in five states (Delaware, Iowa, Massachusetts, New York, and South Carolina) to develop a cohort for subsequent analyses of medical conditions and service utilization. We estimated that over 300,000 Medicaid members in these states had IDD. All members with diagnostic codes for IDD were identified and the three most frequent diagnoses were unspecified intellectual disability, autism or pervasive developmental disorder, and cerebral palsy. The percentage of Medicaid members with IDD ranged from 2.3% in New York to 4.2% in South Carolina. Identifying and characterizing people with IDD is a first step that could guide public health promotion efforts for this population.