The management of dengue fever (DF) has been suggested to be categorized into decision groups A, B, and C; however, its usefulness in predicting mortality is still unclear, and hence we conducted this study to clarify this issue. We conducted a study by recruiting 2,358 patients with DF from the 2015 outbreak in the Chi-Mei Medical Center. Demographic data, vital signs, clinical symptoms and signs, coexisting morbidities, laboratory data, decision groups categorized according to World Health Organization for clinical management of dengue in 2012, and 30-day mortality rates were included for analysis. The overall 30-day mortality rate was 1.4%. The 30-day mortality rates in decision groups A, B, and C were 0%, 0.5%, and 46.2%, respectively. Compared with Group A, there was a higher mortality risk in Group C (odds ratio [OR]: 1,480, 95% confidence interval [CI]: 195-11,200). The area under the curve of the variable of Group C was excellent (OR: 0.92, 95% CI: 0.85-0.99). The sensitivity, specificity, positive predictive value, and negative predictive value for predicting 30-day mortality in Group C were 88.2%, 98.5%, 46.2%, and 99.8%, respectively. This study showed that decision Group C has a good predictive value for 30-day mortality. Further studies including validation in other nations are warranted.