The purpose of this study was to survey staff nurses on their perceived and actual level of knowledge of diabetes mellitus. A convenience sample of 184 professional staff nurses from both inpatient and outpatient settings of a large research-teaching hospital was surveyed. The Diabetes Self-Report Tool (Cronbach's alpha = .91) was used to assess staff nurses' perceptions of knowledge of diabetes mellitus. The Diabetes: Basic Knowledge Test (DBKT; Cronbach's alpha = .79) was used to measure the actual level of knowledge of diabetes mellitus. The data were analyzed with Pearson's correlation coefficients. A moderately low negative correlation (r = -.36, P less than .001) indicated that the staff nurses' perceived knowledge of diabetes mellitus was inversely related to actual knowledge. Subjects were found to have a mean score of 64% on the DBKT. Study findings raise questions as to the adequacy of staff nurse knowledge of diabetes and the ability of staff nurses to assess themselves for knowledge deficits.