Purpose: The purpose of this study was to identify the effects of aroma inhalation therapy on nausea, vomiting, abdominal discomfort, compliance, and colon cleansing for patients having a colon lavage solution before colonoscopy. Methods: This study was performed using a nonequivalent control group post-test design. The study was conducted from 20 Jun to 29 Aug, 2011. The participants were 152 patients (experimental group: 77, control group: 75) who underwent colonoscopy at A hospital, Seoul. Results: The experimental group with the aroma inhalation therapy reported significantly lower nausea and vomiting than did the control group. There were no significant differences in abdominal discomfort, compliance, and colon cleansing between experimental and control group. Conclusion: Use of an aroma inhalation therapy was efficient in nausea and vomiting among patients having a colon lavage solution before colonoscopy. Therefore, aroma inhalation therapy is recommended as an effective nursing intervention for relieving nausea and vomiting among patients having a lavage solution before colonoscopy.