This study uses cross-sectional analysis of huge Medicare hospital discharge databases to test the hypothesis that hospitals with longer inpatient average lengths of stay (ALOS) will have higher healthcare-associated infection (HAI) rates. For 4 carefully defined and important procedures, the authors investigated the relationship between a hospital's HAI rate and the ALOS for patients who did not have such an infection. The authors found a strong positive correlation between the 2 measures. This finding has important implications for improving patient safety and controlling cost. This article offers hypotheses on the causality of the ALOS-infection rate correlation and suggests ways to test those hypotheses.