One of the most common reasons for partial nasal obstruction is nasal septal deviation (NSD). The effect of a partial lateralized nasal obstruction on olfactory bulb (OB) volume remains unclear. Thus, the aim of this study was to investigate the side differences in olfactory function and OB in patients with serious NSD. Sixty-five volunteers were included: 22 patients with serious right NSD and 43 patients with left NSD. The patients' mean age was 22 years. All participants received volumetric magnetic resonance imaging scans of the entire brain and detailed lateralized olfactory tests. The majority of the patients exhibited an overall decreased olfactory function (as judged for the better nostril: functional anosmia in 3%, hyposmia in 72%, normosmia in 25%), which seems to be mostly due to the overall severe changes in nasal anatomy. As expected, olfactory function was significantly lower at the narrower side as indicated for odor thresholds, odor discrimination, and odor identification (P ≤ 0.005). When correlating relative scores and volumes (wider minus narrower side), a significantly positive correlation between the relative measures emerged for OB volume and odor identification, odor discrimination, and odor thresholds. Our study clearly highlights that septal deviation results in decreased olfactory function at the narrower side.