The GESIDA/National AIDS Plan expert panel recommended preferred regimens (PR), alternative regimens (AR) and other regimens (OR) for antiretroviral treatment (ART) as initial therapy in HIV-infected patients for 2018. The objective of this study was to evaluate the costs and the efficiency of initiating treatment with PR and AR.Economic assessment of costs and efficiency (cost-effectiveness) based on decision tree analyses. Effectiveness was defined as the probability of reporting a viral load <50copies/mL at week 48, in an intention-to-treat analysis. Cost of initiating treatment with an ART regimen was defined as the costs of ART and its consequences (adverse effects, changes of ART regimen, and drug-resistance studies) over the first 48 weeks. The payer perspective (National Health System) was applied considering only differential direct costs: ART (official prices), management of adverse effects, studies of resistance, and HLA B*5701 testing. The setting was Spain and the costs correspond to those of 2018. A deterministic sensitivity analysis was conducted, building three scenarios for each regimen: base case, most favourable and least favourable.In the base-case scenario, the cost of initiating treatment ranges from 6788 euros for TAF/FTC/RPV (AR) to 10,649 euros for TAF/FTC+RAL (PR). The effectiveness varies from 0.82 for TAF/FTC+DRV/r (AR) to 0.91 for TAF/FTC+DTG (PR). The efficiency, in terms of cost-effectiveness, ranges from 7814 to 12,412 euros per responder at 48 weeks, for ABC/3TC/DTG (PR) and TAF/FTC+RAL (PR), respectively.Considering ART official prices, the most efficient regimen was ABC/3TC/DTG (PR), followed by TAF/FTC/RPV (AR) and TAF/FTC/EVG/COBI (AR).