Objective To evaluate the prognostic value of human antibacterial peptide LL-37 in elderly patients with sepsis. Methods Elderly sepsis patients over 65-year-old satisfied the diagnostic criteria for sepsis and septic shock admitted to intensive care unit of East Hospital of Tongji University from January 2016 to December 2017 were enrolled (elderly sepsis group). Aged community-acquired pneumonia (CAP) patients hospitalized during the same period were enrolled as a control group for pneumonia, and the aged health check-ups served as a healthy control group during the same period. The peripheral blood LL-37 levels of all patients on the 1st, 3rd, 7th day of admission and the results on the day of physical examination in the healthy control group and on the day of admission in aged CAP group were recorded. C-reactive protein (CRP), arterial blood lactate (Lac), procalcitonin (PCT) were monitored, and acute physiology and chronic health evaluation Ⅱ (APACHEⅡ) and sequential organ failure assessment (SOFA) scores were calculated based on the worst values within 24 hours. The correlation between LL-37 and various indicators was analyzed by Spearman method. According to the 28-day clinical outcome, the elderly patients with sepsis were divided into survival group and non-survival group. The differences in all parameters between the two groups were compared. The statistically significant indicators were analyzed by receiver operating characteristic (ROC) curve, and the predictive value of each indicator for prognosis was evaluated. Results ① A total of 113 elderly patients with sepsis were enrolled in the final analysis, including 67 patients in sepsis group and 46 patients in septic shock group. Thirty-two patients were enrolled as healthy controls and 31 elderly patients with CAP as elderly pneumonia group. The PCT, CRP, Lac, APACHEⅡ and SOFA scores of the patients in the three groups were higher than those of the healthy control group, and they were gradually increased with the severity of infection. There was no significant difference in gender or age among the groups. Compared with the healthy control group, the other three groups had higher LL-37 level after admission, the LL-37 levels in the sepsis group and the septic shock group were decreased with the prolongation of the hospitalization time, and they were lower than the pneumonia group at 7 days after admission [LL-37 (μg/L): 1 403.9±501.9, 1 517.1±676.4 vs. 1 608.4±816.2, both P > 0.05]. It was shown by correlation analysis that the LL-37 level in peripheral blood of elderly patients with sepsis was significantly negatively correlated with APACHEⅡ score (r = -0.329, P = 0.007) and SOFA score (r = -0.344, P = 0.005), but no significant correlation with Lac was found (r = -0.128, P = 0.311). ② The 28-day survival analysis revealed that of the 113 elderly patients with sepsis, 54 (47.8%) survived at 28 days and 59 (52.2%) died. There was no significant difference in gender, age, PCT or CRP levels at 1 day after admission between the two groups. The 1-day Lac, APACHEⅡ and SOFA scores of the patients in the non-survival group were significantly higher than those in the survival group, they were gradually increased with the prolongation of the hospitalization time, and they were significantly higher than those in the survival group at 7 days after admission [Lac (mmol/L): 2.4 (1.4, 4.4) vs. 1.0 (0.8, 1.7), APACHEⅡ score: 21.77±5.85 vs. 13.74±4.99, SOFA score: 9.62±4.78 vs. 3.18±2.71, all P < 0.01]. With the prolongation of admission, there was no significant change in LL-37 level of peripheral blood in the survival group. The LL-37 level in the non-survival group showed a downward tendency, and it was significantly lower than that in the survival group at 7 days after admission (μg/L: 1 277.8±642.6 vs. 1 620.6±461.6, P < 0.05). It was shown by ROC curve analysis that the LL-37 in peripheral blood, Lac, APACHEⅡ score and SOFA score at 7-day of admission of elderly patients with sepsis had predictive value for prognosis, and LL-37 had the best predicted effect for 28-day death, the area under the ROC curve (AUC) of LL-37 was 0.670, 95% confidence interval (95%CI) = 0.513-0.757, when the optimal cut-off value was 1 283.0 μg/L, the sensitivity was 75.7%, and the specificity was 61.5%. Conclusions The expression of LL-37 increased in the early course of the disease in elderly patients with sepsis. However, as the disease progressed and worsened, the level of LL-37 had a decline tendency and was associated with death. The dynamic monitoring of LL-37 combined with APACHEⅡ and SOFA scores had clinical guidance value in predicting the prognosis of sepsis in the elderly.