OBJECTIVES: To estimate the budget impact associated with use of Atazanavir as 1st line treatment in Spanish market of  antiretroviral drugs for VIH patients, on stable and sever health state. METHODS: An economic Model was developed to evaluate the Budget Impact of using Atazanavir (ATV), as 1st line treatment, for VIH patients in the Spanish National Health System perspective, over a 10-year period. Therapies included in the analysis were Darunavir (DRV), Lopinavir (LPV) and Efavirenz (EFV). Patient data were obtained through microsimulation model, with a patient cohort simulated, statistically significant and representative (N= 40,000). The costs were obtained from local databases and were considered pharmaceutical and direct healthcare costs. An annual discount rate assumed was of 3%.The discontinuation rates after AEs assumed for all treatments were: 71.3% from diarrhea, 61.3% from nausea, 28.8% from jaundice, 82.5% from rash and 55% from CNS events. The results of BIM per patient were presented at annual and cumulative level.  RESULTS: Atazanavir use led to differential annual costs per patient after 10 years of treatment of 595€, 209€ and 76€, with respect to DRV, LPV and EFV. The highest savings generated by ATV derived from durable health of 1st line treatment (807€ DRV, 909€ LPV and 1045€ EFV), followed by return to health and durable viral suppression. This savings offset ATV drug cost versus other antiretroviral drugs. CONCLUSIONS: This analysis showed that treatment with Atazanavir for VIH patients, on stable and sever health state, generates net savings for Spanish National Health System: 595€, 209€ and 76€, with respect to DRV, LPV and EFV, in terms of differential annual costs per patient after 10 years treatment.