This study examined unit-level associations of nurse staffing and workload, and the effect of the practice environment on adverse patient events. A secondary analysis was conducted of a longitudinal data set of 23 Army inpatient units from the Military Nursing Outcomes Database. Generalized Linear Mixed Modeling accommodated nested, nonparametric data. Staff category was found to be a significant predictor of medication errors and patient falls, but the relationship varied by unit type. Patient census had no effect on either outcome; however, a higher patient acuity was associated with an increase in both adverse events. The nursing practice environment mediated medication errors but not falls, in all unit types. Skill mix is important; however, additional components of staffing need consideration in producing positive patient outcomes. © The Author(s) 2012.