OBJECTIVES: To characterize psychiatric healthcare utilization and its related costs in a cohort of newly diagnosed ASD individuals. METHODS: A cohort was built using the provincial public healthcare insurance program (RAMQ) databases.  Newly diagnosed subjects with ASD were selected (≥ 2 diagnoses (separate dates) with ICD-9 codes: 299.X, excluding 299.2) between January 1998 and December 2010.  Cohort entry was the date of first diagnosis confirmed by the absence of ASD diagnosis in previous 5 years.  Participants aged ≥ 26 years or those not covered by the RAMQ drug plan in the year preceding cohort entry were excluded.  Demographic and clinical patient characteristics were done at cohort entry.  Healthcare utilisation associated with a psychiatric diagnosis (physician and emergency room visits, hospitalisations), psychoactive drug use (anticonvulsants, antipsychotics, antidepressants, anxiolytics, ADHD drugs) and total cumulative costs were assessed across 5 years of follow-up.  RESULTS: A number of 1227 patients among the total cohort had 5 years of follow-up (male: 80.3%; median age: 7 years).  In the 1-year following diagnosis, the mean number of visits to general practitioners was 1.6 whereas psychiatry related visits to specialists were 7.2 which decreased over time.  Psychoactive drug utilisation was initially present in 49.3% of participants, and increased to 53.2% at 5 years.  The psychiatric hospitalization rate was 10.4% in the 1-year of follow-up with the highest rates seen in adolescents (20%) and young adults (24%).  Cumulative costs of psychiatric health services and psychoactive drugs at 1-year of follow-up were $547,568CAD and $600,433CAD, respectively.  Those costs were at $216,827CAD and $934,381CAD at 5-years.  CONCLUSIONS: While cost related to psychiatric healthcare services decreased by more than half in the 5 years of follow-up, drug costs rose by 56%.  Access to long term care and monitoring among the ASD population is discussed.