Background P300 event-related potentials can be used to measure auditory processing speed, working memory, and attention. Purpose The purpose of the study was to compare P300 event-related potentials in normal-hearing adults with those of adults with Type II diabetes mellitus. Research Design A two-group (with diabetes and controls) comparative study (age- and sex-matched) with a nonprobability sampling method was used. Study Sample Sixty-four adult participants (32 with diabetes, 32 without diabetes) between the ages of 23 and 60 years participated. Data Collection and Analysis Pure-tone audiometry was performed to ensure participants had pure-tone averages of ≤ 25 dB HL. Folstein Mini-Mental State Examinations were conducted, which ensured absence of cognitive impairment. Blood glucose levels were measured immediately prior to P300 testing, after which amplitude and latency results were captured. Descriptive analysis was used to calculate mean, standard deviation, median, and 25th and 75th percentiles. To study differences between adults with and without diabetes as well as the effect of glucose, linear mixed-model regression analyses were performed when left and right ears were combined, and simple linear regression analyses were performed when left and right ears were analyzed separately. Results For P300 latency results, a significant statistical difference (p < .001) was observed between participants with and without diabetes (352.46 ms, SD = 36.36; 314.09 ms, SD = 32.08), respectively. A significant statistical difference (p < .001) in amplitude was observed between participants with and without diabetes, respectively (12.10 μV, SD = 3.70; 15.08 μV, SD = 2.82). Glucose was a key moderator of only amplitude after adjusting for diabetes status. Glucose had no effect on amplitude and latency for adults without Type II diabetes mellitus (DM). Conclusions Type II DM decreases amplitude and increases latency; in addition, adults with Type II DM, attention, and working memory, as denoted by P300 amplitude, may deteriorate with increased glucose levels on the day of testing.