INCMNSZ is one of MexicoÂ´s National Institutes of Health. Complicated diseases are referred from all over Mexico. We sought to compare characteristics of patients with Average hospital Length Of Stay (ALOS, <32 days) and Prolonged Length of stay (PLOS, > 32 days [mean LOS + two standard deviations]), to identify factors associated to poor outcomes and PLOS. Methods: analysis of hospitalizations at INCMNSZ (2000-2015), comparing ALOS and PLOS using descriptive and inferential statistics. Results: 80,330 hospitalizations (856,953 bed-days), 3.9% were PLOS (17.4% of total bed-days). Mean hospital LOS was 9.2 days in ALOS and 47 in PLOS. Average age was 50.9 years in ALOS and 46.8 in PLOS (p<0.0001). Most common discharge diagnosis: pneumonia in ALOS and peritonitis in PLOS. Surgery was performed in 40.3% of ALOS and 57.98% of PLOS (p<0.0001); most common was cholecystectomy in ALOS and laparotomy in PLOS. Socioeconomic status was predominantly low (73.9% in ALOS vs 79% in PLOS, p<0.0001). In-hospital mortality was 2.4% (2.3% in ALOS vs 4.7% in PLOS, p<0.0001). Average age of death was 62.6 years in ALOS vs 52 in PLOS (p<0.0001). Most common cause of death: complicated diabetes mellitus in ALOS and acute leukemia in PLOS. Deaths in ALOS had surgery in a lower proportion (19.4% vs 43.5%, p<0.0001). Conclusions: poor outcomes and PLOS were associated with abdominal, metabolic and hematologic diseases, surgery, and low socioeconomic status. Mortality was greater in PLOS and related to surgery. More studies to identify correctable risk factors for poor outcomes and PLOS are needed.