Self-injurious behavior (SIB) is a chronic disorder that often begins in early childhood; however, few studies have examined the onset of SIB in young children. This preliminary study reports on the identification, assessment, and observation of SIB in 32 children who had begun to engage in SIB within the previous 6 months. Participants were under 5 years of age and presented with or were at risk for intellectual and/or developmental disabilities. Assessment measures included parental interviews, developmental and language measures, standardized measures of problem behavior, and direct observations conducted in the home. Results indicated that for most children, SIB emerged prior to age 1 year, and most children developed multiple topographies of SIB and other problem behaviors. Multiple measures were useful in identifying SIB and in characterizing the behavior by topography, frequency, and severity. Findings from the examination of child communication in relation to SIB were inconclusive. Results are discussed in relation to theories of SIB emergence and previous observational studies of young children with SIB. (Contains 5 tables.)