Background: Everolimus is a molecularly targeted drug for renal cell carcinoma. It is also approved for pancreatic neuroendocrine tumors ("PNET") and tuberous sclerosis complex ("TSC") in Japan and frequently associated with stomatitis, one of the most common adverse reactions. However, the mechanism of the onset of stomatitis has not been elucidated, and no reports have been published on appropriate prophylaxis against everolimus-induced stomatitis. Method: In the Department of Urology, Yamagata University Hospital, gargling with sodium azulene sulfonate has been used prophylactically since October 2010, when oral treatment with everolimus was first administered at our hospital. In this study, the preventive effect of gargling with sodium azulene sulfonate on stomatitis was evaluated in 21 patients receiving everolimus. Result: The incidence of stomatitis was 53.3% (Grade 3/4: 0%) in the intervention group and 83.3% (Grade 3/4: 16.7%) in the non-intervention group; patients treated in October 2010 or earlier. The grade of severity was significantly lower in the intervention group (Median grade: 1 vs. 2, p<0.05; Mann-Whitney’s U test). However, there was no difference in the time of onset between the two groups. Discussion: Sodium azulene sulfonate effectively prevented stomatitis probably because it not only has anti-inflammatory and mucosal protective effects, but also promotes wound healing. Further detailed analysis will be required in more patients.