Elderly patients have been shown to have an increased risk of acquiring nosocomial infection per hospital admission. To determine if the length of stay accounts for this risk, daily infection rates were computed per decade of life and rates for patients over and under 60 were compared using risk ratios. Four thousand thirty-one nosocomial infections in 2,567 patients were identified for a 1980 through 1984 admission cohort in an acute-care hospital. The daily infection rates were 0.59 percent in patients over age 60 and 0.40 percent in younger patients (relative risk = 1.49). The daily incidences of urinary tract infections, respiratory infections, and septicemias were all significantly increased in elderly patients with risk ratios of 2.78, 2.07, and 1.36, respectively. Further analysis revealed that elderly patients experienced significantly more nosocomial infections for each day of hospitalization after Day 7. These data show that elderly patients experience an increased daily rate of nosocomial infection, and suggest that efforts be directed at identifying clinical conditions that predispose this population to hospital-acquired infections.