This study analyzes the effectiveness of adding lactate levels to the Rapid Emergency Medicine Score (REMS) scoring system to better predict short-term mortality and outcomes in patients over 40 years of age who present to the emergency department (ED) with dyspnea. This prospective observational study recruited all consecutive patients with shortness of breath as their chief complaint. Patients were evaluated using REMS and categorized in the ED as low-, intermediate-, or high-risk. In-hospital outcomes and the survival rates of the patients were recorded. The patients’ REMS points and lactate levels were analyzed together to elicit the REMS+L scores used to predict mortality and outcomes. A total of 1044 patients were included in the study. The majority (64.8%, n = 677) of the patients received diagnoses related to the respiratory system, 9.9% (n = 103) with the cardiovascular system, and 25.3% (n = 264) with nonspecific diagnoses. A total of 31% (n = 324) of the patients were hospitalized, while the majority (78%, n = 253) were admitted to an intensive care unit. A total of 104 (10%) died within 28 days, with 23 of those deaths (2.2%) occurring within 2 days. The diagnostic accuracies of lactate, REMS, and REMS+L values were calculated using receiver operating characteristics (ROC) analysis and revealed that the REMS+L score (p < 0.001) was more accurate than the lactate measurements (p < 0.001) and REMS score (p < 0.001) in predicting short-term mortality. The REMS+L score (p < 0.001) was superior to the REMS (p < 0.001) and lactate values (p < 0.001) in predicting mortality. Adding lactate measurements to REMS in patients over 40 years of age who present to the ED with shortness of breath appeared to yield more accurate estimates than using REMS and lactate values alone when determining two-day mortality.