Background: Colonoscopy-associated anxiety can cause different complications and therefore, anesthesia, opioids or tranquilizers should manage it. Objectives: The aim of this study was to evaluate the effects of inhalation of aromatherapy with lavender on anxiety amongst patients undergoing a colonoscopy. Methods: This randomized controlled clinical trial was conducted on patients referred to the colonoscopy center of Qaem (PBUH) Hospital, Mashhad, Iran. Fifty patients were randomly allocated to either a control group or an aromatherapy group, 25 patients in each one. Once admitted to the hospital, the patients completed a demographic questionnaire and Spielberger's State Anxiety Inventory. Afterward, for ten minutes, the patients in the aromatherapy group inhaled three drops of pure lavender essential oil, which was sprayed, by using a nebulizer, while patients in the control group inhaled five milliliters of distilled water in the same way as the patients in the experimental group. Finally, the patients in both groups were asked to recomplete the Spielberger's State Anxiety Inventory both before and after undergoing a colonoscopy. The SPSS software (v. 19.0) was used to analyze data by the Chi-square, the Fisher's exact test and the independent sample t-tests as well as the repeated measures analysis of variance at a significant level of less than 0.05.