Use of supplemental RNs (SRNs) is common practice among US hospitals to fill gaps in nurse staffing. The objective of this study was to examine the relationship between use of SRNs and patient outcomes. Multilevel modeling was performed to analyze hospital administrative data from 19 hospital units in a large tertiary medical center for the years 2003 to 2006. Patient outcomes included in-hospital mortality, medication errors, falls, pressure ulcers, and patient satisfaction with nurses. Use of SRNs ranged from 0% to 30.4% of total RN hours per unit quarter. Among 188 of the 304 unit quarters in which SRNs were used, the average SRN use was 9.8% in non-ICUs and 6.4% in ICUs. All observed effects of SRN use on patient outcomes were nonsignificant. Use of SRNs was substantial and varied widely by unit. No evidence was found that links SRN use to either adverse or positive patient outcomes.