Background/Aims: Chinese herbal medicine Qing-Dai (also known as indigo naturalis) has been used to treat various inflammatory conditions. However, not much has been studied about the use of oral Qing-Dai in the treatment for ulcerative colitis (UC) patients. Studies exploring alternative treatments for UC are of considerable interest. In this study, we aimed at prospectively evaluating the safety and efficacy of Qing-Dai for UC patients. Methods: The open-label, prospective pilot study was conducted at Keio University Hospital. A total of 20 patients with moderate UC activity were enrolled. Oral Qing-Dai in capsule form was taken twice a day (daily dose, 2 g) for 8 weeks. Results: At week 8, the rates of clinical response, clinical remission, and mucosal healing were 72, 33, and 61%, respectively. The clinical and endoscopic scores, CRP levels, and fecal occult blood results were also significantly improved. We observed 2 patients with mild liver dysfunction; 1 patient discontinued due to infectious colitis and 1 patient discontinued due to mild nausea. Conclusion: This is the first prospective study indicating that oral Qing-Dai is effective for inducing remission in patients with moderate UC activity and can be tolerated. Thus, Qing-Dai may be considered an alternative treatment for patients, although further investigation is warranted.