OBJECTIVES: Characterize the temporal course of psychiatric healthcare utilization and related costs in a cohort of newly diagnosed ASD individuals. METHODS: A cohort was built using RAMQ databases.  Newly diagnosed subjects with ASD were selected (≥ 2 diagnoses ICD-9 codes: 299.X, excluding 299.2) between January 1998 and December 2010.  Cohort entry was the date of first diagnosis confirmed by absence of ASD diagnosis in previous 5 years.  Participants aged ≥ 26 years, those without full RAMQ drug plan coverage for 5 years after cohort entry, or not covered in the year preceding cohort entry were excluded.  Demographic and clinical characteristics were done at cohort entry.  Descriptive use of psychiatric healthcare care (medical visits, hospitalizations, psychoactive drug use (anticonvulsants, antipsychotics, antidepressants, anxiolytics, ADHD drugs, anticholinergics, lithium) was assessed during a 5-year follow-up period. Mean costs per patient was estimated at 1- and 5-year period of follow-up. Bootstrap analyses were used to assess the differences in costs between 1-year and 5-year follow-up. RESULTS: A total of 1,227 individuals were identifed (male: 80.3%; median age: 7 years).  In the 1-year period after diagnosis, mean number of psychiatric medical visits was 7.5 ± 14.6 with mean cost per patient of $653CAD (95%CI: 582-723); those values reduced to 2.1 ± 4.2 and $205CAD(175-235), respectively at 5-years. Psychiatric hospitalization rate was 10.4% with mean cost per patient $9,717CAD(7,253-12,180) at year 1; the rate and cost reduced to 3.7% and $4259 CAD(2,395-6,123) at 5-years.  Psychoactive drug utilization was initially present in 49.3% of subjects, and increased to 53.2% at 5-years.  Associated mean drug costs per patient increased from $622CAD(539-706) to $889CAD(786-991), in this time period. CONCLUSIONS: While costs for psychiatric medical visits and hospitalizations decreased by more than half over 5 years, psychoactive drug costs rose by 43%.  Access to long term care and monitoring among the ASD population is discussed.