The objective of this study was to assess the influence of overcrowding on health care quality provided by emergency departments (ED). The study was carried out in an urban, university tertiary care hospital. All patients seen at the internal medicine unit (IMU) of the ED who returned during the following 72 hours, and those who died in the ED rooms were included in the study. During a consecutive period of 2 years (104 weeks), we prospectively quantified the number of weekly visits, revisits and deaths. We calculated revisit and mortality rates (in respect of percentage of all visited patients) for each week. Correlation between the number of weekly visits, and revisit and mortality rates was assessed using a simple linear regression model. We consigned 81,301 visits, 1137 revisits and 648 deaths; mean (+/- SD) number of weekly visits, revisits and deaths were 782 (68), 10.93 (3.97) and 6.23 (3.04) respectively; weekly revisit rate was 1.40% (0.48%) and weekly mortality rate was 0.79% (0.36%). We observed a significant, positive correlation between mortality rates and weekly number of visits (p = 0.01). Although a similar trend was also found for revisit rates, such an increase did not reach statistical significance (p = 0.06). It is concluded that since revisit and mortality rates constitute good health care quality markers, present data demonstrate that ED overcrowding implies a decrease in the health care quality provided by it.