Mesalazine is a drug used to treat ulcerative colitis and Crohn's disease, and is known to rarely cause lung injury. We show herein a unique case who developed this drug-induced injury. A 17-year-old boy presented with fever and anorexia after administration of mesalazine. Computed tomography showed extensive ground-glass opacities with peripheral distribution in both lungs. He had general weakness, but had no respiratory symptoms such as cough and dyspnea. With prednisolone, which is primarily aimed at controlling ulcerative colitis, the extensive opacity in both lungs were improved. All patients with this drug-induced lung injury reported to date have had respiratory symptoms, but this patient had no subjective respiratory symptoms and had no abnormalities in respiratory rate and oxyhaemoglobin saturation. Although very rare, we do believe that this clinical course will provide some suggestive information on treatment for patients with similar course in the future.