Appropriate antimicrobial therapy is effective for severe infections caused by Acinetobacter baumannii, but efficacy for other Acinetobacter species remains to be established. The current study was designed to determine whether appropriate antimicrobial therapy reduces the mortality of patients with Acinetobacter nosocomialis bacteraemia. A 9-year retrospective study of 266 patients with monomicrobial A. nosocomialis bacteraemia was conducted at a large teaching hospital in Taiwan. Multivariable analysis was performed to evaluate the impact on 14-day mortality according to clinical characteristics, severity of disease and use of appropriate antimicrobial therapy. The influence of APACHE II score on the impact of appropriate antimicrobial therapy was analysed by including an interaction term. The overall 14-day mortality was 9.4%. Multivariable analysis revealed that APACHE II score was the only factor significantly associated with mortality (odds ratio, 1.18; 95% confidence interval, 1.11-1.25; p <0.001). Appropriate antimicrobial therapy was not associated with reduced mortality regardless of disease severity. In the subgroup analyses in patients with different clinical conditions, APACHE II score was consistently an independent factor for 14-day mortality, and appropriate antimicrobial therapy did not affect the mortality in any group. In conclusion, severity of disease, based on the APACHE II score, was the independent risk factor for 14-day mortality for patients with monomicrobial A. nosocomialis bacteraemia, even in different clinical conditions. In contrast, appropriate antimicrobial therapy did not reduce the 14-day mortality. The result highlighted a different effect of appropriate antimicrobial therapy on infections caused by two phenotypically undifferentiated Acinetobacter.