Autism Spectrum Disorder (ASD) and Attention Deficit Hyperactivity Disorder (ADHD) frequently co-occur and show high genetic correlation. With the introduction of DSM-5, there is a new concept of an ASD and/or ADHD spectrum (ASD/ADHD). This study aimed to identify predictors of severity and need of healthcare within this spectrum. 39 families with multiple individuals affected by ASD/ADHD were recruited from a psychiatric clinic. Diagnoses, functional and demographic characteristics were retrieved from journals while hospital admissions were identified in the Danish health register. An estimated fraction of 31% ASD/ADHD patients had never been hospitalized and 35% remained undiagnosed despite hospitalization. Cluster analysis identified trajectories that discriminate age of diagnosis, educational attainment to degree of severity, need of hospitalization and genetic risk.