Objective: This study aims to delineate the characteristics of severe self-injurious behaviors (SIB) in a cohort of children with autism and unspecified intellectual developmental disorder (UIDD) (intellectual disability) and examine potential risk factors for developing SIB. Methods: A retrospective chart review studied characteristics of severe SIB in 30 children with autism spectrum disorder (ASD) and UIDD referred to a tertiary care center. Characteristics examined include genetic syndromes, brain MRI abnormalities, verbal ability, adaptive functioning, SIB frequency and severity, age of onset, number of psychopharmacological agents, irritability, hyperactivity, stereotypy, psychiatric and physical comorbidities, among others. Descriptive and bivariate analysis were applied to explore potential relationships between factors. Results: Children with severe SIB exhibit this behaviour with high frequency, inflicting moderate to severe injury. Most children in the study sample are non-verbal and have ASD (93.3%; n = 28) with psychiatric (96.7%; n = 29) and physical (90%; n = 27) comorbidities. Overall SIB improvement using the Clinical Global Impression, Improvement Score (CGI-I) was 3.0 (minimally improved). A minority were much or very much improved following appropriate intervention. Conclusions: The severity of SIB is much higher in this sample than previously noted in the literature. Severe SIB is associated with ADHD, early onset mood disorders, tics, avoidant restrictive food intake disorder and Obsessive-Compulsive Disorder.