Self-injurious behavior (SIB) is a devastating condition associated with intellectual and developmental disabilities (IDD). Efforts to understand its development are focused on early childhood when the behavior first emerges. Limited prevalence data on SIB during early childhood are currently available. The purpose of this study was to determine the prevalence of SIB in a sample of children with IDD between the ages of 18 and 72 months and associated demographic, diagnostic, and behavioral characteristics. The prevalence rate was 19.1% for SIB including head banging, self-biting, and hitting self. Factors associated with SIB in adults, such as severe intellectual disability; visual, hearing, or mobility impairment; or diagnoses of autism, cerebral palsy, or seizure disorder were not associated with SIB in these children. However, children with SIB had higher rates of Hurtful to Others than the contrast group. Issues related to the characterization of SIB in young children are also discussed. (Contains 3 tables.)