Fit testing of respirators has been recommended, legislated, and implemented in many countries. In the United States, the National Institute for Occupational Safety and Health advocates a fit check, or seal check, before donning a respirator for each fit test. The aim of this study was to investigate the relationship between respirator fitness and performance of the fit check upon donning a respirator. Eighty-four first-year undergraduate nursing students were selected for this study; they were divided randomly into four groups. None had performed a fit test or fit check before being recruited for this study. The real-time self-developed fit test method was used to measure the ambient air particle concentration outside and inside the respirator. Results showed significant differences between groups trained to perform the fit check and those untrained. The overall fit factors were higher in the two groups trained to perform the fit check. The overall fit factors were lower in the two groups not trained to do the fit check. N95 respirators are designed to form a tight seal against wearers’ faces. Each time workers don respirators, they should evaluate proper fit. Training wearers in how to fit check increases the likelihood that they will adjust respirators to proper fit. Copyright © 2012 American Association of Occupational Health Nurses, Inc.