Objectives: To compare health care utilization patterns and cost among insured adults with autism spectrum disorder (ASD), adults with attention-deficit and hyperactivity disorder (ADHD), and adults with neither condition (general population [GP] controls). Method: We conducted a case-control study among adults (q18 years) who were members of Kaiser Permanente Northern California (KPNC) for at least 9 months each year from 2008 to 2012. Cases (N = 1507) were adults with an ASD diagnosis (ICD-9-CM 299.0-299.8) recorded in the electronic medical record on at least two separate occasions by December 31, 2012. Two control groups, adults with ADHD (N = 9042) defined by ICD-9-CM code 314 and GP (N = 15,070), were randomly selected and frequency matched to cases on gender and age. Health care utilization and cost data were obtained from KPNC databases for the year 2012. Results: Compared with adults with ADHD, adults with ASD had significantly higher utilization of outpatient visits for primary care (74.2% vs. 66.6%), mental health (43.3% vs. 33.2%), and laboratory services (60.9% vs. 54.4%). Hospitalizations for ambulatory care sensitive diagnoses (5.4% vs. 2.3%) were less frequent overall but more common among adults with ASD than with ADHD. Group differences were larger comparing adults with ASD with GP controls. Gynecology visits and screening for cervical cancer were significantly less common among women with ASD than among women with ADHD (35% vs. 50%) or GP (35% vs. 49%). Total annual mean healthcare costs for adults with ASD were 20% higher than costs for adults with ADHD and double costs for GP. Conclusion: Adults with ASD had significantly higher rates of utilization across most health care service areas compared with adults with ADHD or GP; however, women with ASD were significantly less likely to have gynecology visits and have screening for cervical cancer.