Diagnoses of autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD) are increasingly being made in adulthood. However, assessments can fail to address the diverse range of problems that patients have experienced. The current study applied an early symptomatic syndromes eliciting neurodevelopmental clinical examinations (ESSENCE) framework to explore retrospectively reported childhood developmental and behavioral problems. It examined if adult ASD and ADHD patients would show problems outside those reflected in the respective diagnostic criteria, and also if these patient groups would show more extensive childhood problems than other psychiatric patients. Parents of adults with ADHD ( n = 130), ASD ( n = 57), coexisting ADHD and ASD ( n = 38), and other psychiatric disorders ( n = 56) reported on a range of childhood problems. Descriptions of the ADHD, ASD, and ADHD+ASD groups reflected greater impairment than descriptions for patients with other psychiatric disorders in most problem areas. Although differences were observed between ADHD and ASD patients in the core diagnostic areas, these syndromes also shared a number of childhood difficulties. The ESSENCE approach can assist in understanding the symptom history of adult ADHD and ASD patients and can be helpful to distinguish their childhood experiences from other psychiatric patients' experiences.