Mesalazine is a main medicine for treatment of ulcerative colitis. Most patience of left-sides and total colitis receive oral mesalazine for many years. Currently, there is a little information about the tolerability and safety of long-term use of mesalazine. The eosinophilic pneumonia, organizing pneumonia, and nonspecific interstitial pneumonia are very rare adverse effects of ulcerative colitis treatment with mesalazine. The article presents case of the development interstitial lung disease induced by mesalazine under long-term maintenance treatment for three years in the young patient with ulcerative colitis. It shows the difficulties in diagnosing this disease due to the work-long low-grade fever in manifestation of pneumonitis, the similarity of clinical and radiological manifestations (diffuse bilateral pattern in chest imaging). The article demonstrates the limitations of modern laboratory and instrumental diagnostic methods for the differentiation of disseminated lesions of the lung tissue, and shows the importance of elimination treatment of mesalazine-induced pneumonitis.