Objective The current study investigated the relationship between adiposity and P3 latency. Methods Fifty-one adults in two age groups (18-25 and 65+) participated. Odor stimuli were delivered via olfactometer as participants focused on a computer screen. Each stimulus was followed by presentation on the screen of four odor identification choices. Participants attempted identification by button press. Olfactory event-related potentials (OERPs) were recorded. BMI and waist circumference were measured as indicators of adiposity. Results In bivarate analyses with all participants included, positive correlations for P3 latency with both BMI and waist circumference were observed, indicating that as adiposity increased latencies also increased. When each age group was separately examined, correlations between adiposity measures and latency remained statistically significant for older adults. Furthermore, ApoE ε4 allele status was examined. Latencies remained positively correlated with adiposity in older adult ε4 carriers; but not in non-carriers. Conclusions This study indicates that adiposity predicts olfactory processing speed in older adults, specifically in ε4 carriers. Significance The results suggest that olfactory processing speed may be a useful measure for detecting and following the effects of adiposity on brain integrity and cognitive function in those at genetic risk for AD.