A number of cross-sectional studies report extensive use of psychiatric services and high healthcare costs in autistic youths. However, little is known about how the use of these services evolves from the time of diagnosis, as children grow up. Our objectives were to investigate the use, costs, and predictors of psychiatric services following autism spectrum diagnosis. We built a cohort of 1227 newly diagnosed autism spectrum individuals identified in the Quebec (Canada) Régie de l’assurance maladie du Québec administrative database (January 1998 to December 2010). Mean number and cost per individual of psychiatric healthcare use (hospitalizations, medical visits, psychoactive drug use) were calculated yearly for 5 years following autism spectrum diagnosis. Mean number of psychiatric visits decreased over time by more than threefold (7.5 vs 2.1 visits) from year 1 to year 5, whereas psychoactive drug use increased from 16.0 to 25.2 claims. Psychiatric hospitalizations decreased during follow-up, but still represented the greatest costs per individual (CAD9820 for year 1; CAD4628 for year 5). Antipsychotics represented over 50% of drug costs. Mixed-effect model with repeated measures showed that previous psychoactive drug use was the strongest predictor of greater psychiatric healthcare cost during follow-up (odds ratio: 9.96; 95% confidence interval: 7.58–13.10). These trends contrast with guidelines advocating cautious prescribing of antipsychotics with periodical re-assessment of their benefit.