INTRODUCTION. Frailty is a clinical syndrome that arises due to age-related decline, diseases, malnutrition and lifestyle. Two major perspectives on frailty exists: frailty as a phenotype and frailty as an accumulation of deficits. The two types are measured by Fried???s Phenotype (FP) and the Clinical Frailty Scale (CFS), respectively. The aim of this study was to investigate which model best predicts 90-day mortality in elderly patients acutely admitted to an emergency department in Denmark. METHODS. This study comprised a prospective cohort with the following inclusion criteria: age 65 years, acute admission and admission >24 h. Bispebjerg Hospital, Odense University Hospital and Hospital of Southwest Jutland participated in the study. The FP and the CFS were measured in all patients. Descriptive statistics, relative risk (RR), odds ratio (OR), risk difference and receiver-operating characteristics (ROC) analysis were performed. The outcome was 90-day mortality. RESULTS. A total of 1,030 patients participated (mean age: 78.2 years, 54% female). Among these, 221 were frail by the FP (score > 3) and 555 participants were frail by the CFS (score > 5). Within 90 days, 128 died. The analyses revealed significant associations between frailty and 90-day mortality. For the FP, the RR was 2.67 (95% confidence interval (CI): 1.93-3.69), p < 0.001; and for the CFS, the RR was 4.12 (95% CI: 2.65-6.42), p < 0.001. The adjusted OR for the CFS was 4.38 (95% CI: 2.68-7.13); for the FP, 3.88 (95% CI: 2.51-6.01). CONCLUSION. A significant association existed between frailty and 90-day mortality in the Danish cohort. However, the CFS is a better predictor of 90-day mortality the FP. Even so, the CFS still has a lack of sensitivity and specificity.