To evaluate lactate and the Quick Sepsis-Related Organ Failure Assessment (qSOFA) and compare their ability to predict 30-day mortality in patients treated for infection in emergency departments (ED). Prospective multicenter observational cohort study. We enrolled a convenience sample of patients aged 18 years or older attended in 71 Spanish ED from October 1, 2019, to March 31, 2020. Each model's predictive power was analyzed with the area under the receiver operating characteristic curve (AUC), and its values of sensitivity (Se), specificity (Sp), positive predictive value (PPV) and negative (NPV). A total of 4439 patients with a mean (SD) age of 18 years were studied; 2648 (59.7%) were men and 459 (10.3%) died within 30 days. For 30-day mortality, the AUC-COR obtained with the qSOFA = 1 model plus 2 mmol/l lactate was 0.66 (95% CI, 0.63-0.69) with Se: 68%, Es: 70% and NPV:92%, while qSOFA = 1 obtained AUC-COR of 0.52 (95% CI, 0.49-0.55) with a Se:42%, Es:64% and NPV:90%. To predict 30-day mortality in patients presenting to the ED due to an episode of infection, the qSOFA =1 + lactate≥2 mmol/L model significantly improves the predictive power achieved individually by qSOFA1 and becomes very similar to qSOFA≥2.