Aim: To determine associated factors related to overcrowding in the emergency room at the Dos de Mayo National Hospital. Design and Study Population: Analytical, observational, quantitative study. Variables and Measurements: Demand was determined. Times in triage, admission, medical care, auxiliary tests and stay were measured to assess the efficiency of the processes. Finally, the existing free beds in hospitalization were determined. Statistical Analysis: Bivariate analysis and multivariate logistic regression were used. P <0,05 was considered significant. Results: The average age of the patients was 48.79 (SD ? 19,50). Demand in these last two years has increased around 1.57% monthly and 18.85% annually. The total care time was 286,12 minutes ? 36.45 (4,76 hours ? 0.60). The increase in demand (input), and the deficiency of health care processes were not associated with overcrowding, according to bivariate analysis (p> 0.05). Contrary, to the output processes, were closely associated with overcrowding according to the multivariate logistic regression analysis. X2: 48.46, OR: 18.50, p: 0.00. Conclusions: The absence of available beds in hospitalization of medicine service is associated with overcrowding, it was found that the availability of less than 8 beds / day in hospitalization rooms leads to 80% overcrowding per day/ month of the emergency room.