In clinical settings, olfactory testing is usually performed bilaterally; thus, unilateral olfactory loss may go unnoticed. The aims of this study were to evaluate 1) whether patients presenting with self-reported olfactory disorders demonstrate significant side differences in odour perception, depending on the prevalance of measured unilateral disorder, and 2) to evaluate the existing testing procedure. In 518 patients presenting with olfactory disorders, olfactory testing was performed using the "Sniffin' Sticks" test battery (consisting of a threshold, discrimination, and odour identification test) examining each nostril separately. According to the history and results from the clinical examination, olfactory disorders were classified as related to trauma, sinunasal disease, upper respiratory tract infection (URTI), tumour, congenital, idiopathic, and other. A difference of three or more points in one of the subtests or six or more points in the composite olfactory test score was considered a side difference. In almost one quarter of all presenting patients (23.4%), a side difference was detected. To not to miss lateralized disorders, we recommend testing each nostril separately. Depending on the presence or absence of a significant difference, testing then can be continued birhinally or separately for each nostril.