Evidence suggests that substance use disorders (SUD) tend to be underdiagnosed in psychiatry. The objective of this study was to investigate whether drug and alcohol screening is a clinical routine in the assessment of two prominent neurodevelopmental disorders, namely ADHD and autism spectrum disorder (ASD). We surveyed drug and alcohol screening routines in 34 general child and adolescent (only practice for adolescents, not children, was assessed) and 29 adult psychiatric outpatient departments in Stockholm County, Sweden. Structured telephone interviews mapping SUD screening procedures were conducted with department representatives in charge. Only a minority of child and adolescent departments regularly used SUD screening questionnaires (6 %) in ADHD and ASD assessment, while this was more common in adult psychiatry (55 %). Urine/blood-based toxicology tests were always used in 28 % and sometimes or in case of clinical suspicion in 38 % of the adult units. Such tests were used sometimes or in case of clinical suspicion in 15 % of the child psychiatric departments, but never routinely. Findings reveal that screening for SUD in ADHD and ASD is not an integral part of routine clinical assessments in psychiatry, although increasingly an integral part of many clinical guidelines. Thus, SUD might be underdiagnosed in neurodevelopmental disorders, which could be particularly true for child and adolescent psychiatry settings.