OBJECTIVES: The aim of this study was to estimate the budget impact of extending the access to risperidone for adults with ASD and aggressiveness in SUS.  METHODS: Adopting the Ministry of Health’s (MoH) perspective, with direct medical costs, we estimated the budget impact for a 3 year time horizon. The target Brazilian population, with ages between 18 and 74, was based on epidemiological data. Data about dosage, resource use and clinical features were extracted from literature. Drug and test costs were based on the prices reimbursed by the MoH. Uncertainty was handled with one-way and probabilistic sensitivity analysis (Monte Carlo simulations) on the key parameters: ASD prevalence, aggressiveness incidence, non-pharmacologic interventions failure, SUS coverage rate, hyperprolactinemia incidence, risperidone dosage and prolactin testing. Beta and gamma probability distributions were adopted according the parameter profile.  RESULTS: In the base-case, we estimated a total budget of R$ 12,164,094.18 to treat 97,622 adults over a 3 year time horizon. Risperidone represented 37% of the budget. The maximum amplitude identified in the one-way sensitivity analysis was related with the ASD prevalence variation (R$ 19 million variation). After 1,000 simulations, the mean total budget estimated was R$ 13,238,773.00 (95%CI: R$ 12,761,473.00 to R$ 13,716,073.00). Assuming a R$ 10 million a year  budget impact threshold, approximately 98%, 97% and 93% of the simulations would be within the threshold in the first, second and third year, respectively. CONCLUSIONS: Risperidone coverage expansion for adults with ASD and aggressiveness can reflect an average annual budget impact of approximately R$ 4.5 million, with over 90% probability of it being below the R$ 10 million a year threshold over the next 3 years.