OBJECTIVE: There is growing evidence that low nurse staffing jeopardizes quality of patient care. The objective of the study was to determine whether low staffing level increases the infection risk in critical care. DESIGN: Observational, single-center, prospective cohort study. SETTING: Medical intensive care unit of the University of Geneva Hospitals, Switzerland. PATIENTS: All patients admitted over a 4-yr period. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Study variables included all infections acquired in critical care, daily nurse-to-patient ratio, demographic characteristics, admission diagnosis and severity score, comorbidities, daily individual exposure to invasive devices, and selected drugs. Of a cohort of 1,883 patients totaling 10,637 patient-days, 415 (22%) developed at least one healthcare-associated infection while in critical care. Overall infection rate was 64.5 episodes per 1000 patient-days. Infected patients experienced higher mortality with a longer duration of stay both in critical care and in the hospital than noninfected patients (all p <.001). Median 24-hr nurse-to-patient ratio was 1.9. Controlling for exposure to central venous catheter, mechanical ventilation, urinary catheter, and antibiotics, we found that higher staffing level was associated with a >30% infection risk reduction (incidence rate ratio, 0.69; 95% confidence interval, 0.50-0.95). We estimated that 26.7% of all infections could be avoided if the nurse-to-patient ratio was maintained >2.2. CONCLUSIONS: Staffing is a key determinant of healthcare-associated infection in critically ill patients. Assuming causality, a substantial proportion of all infections could be avoided if nurse staffing were to be maintained at a higher level.