A 35-year-old male patient visited our clinic due to fever, productive cough aggravated 1 hour after swimming and a weight loss of 8 kg during a period of 2 months. He began to swim 3 months ago. Simple chest radiography was normal, but chest high-resolution computed tomography (HRCT) showed diffuse bilateral ground-glass opacities with centrilobular nodules. Analysis of bronchoalveolar lavage fluid showed increased T-lymphocyte counts with a reversed CD4＋/CD8＋ ratio without any endobronchial lesion. His symptoms and HRCT findings disappeared after stopping swimming. Based on these results, he was diagnosed as having hypersensitivity pneumonitis. To the best of our knowledge, this is the first report of hypersensitivity pneumonitis that occurred in an indoor swimming pool in this country. (Korean J Asthma Allergy Clin Immunol 2010;30:228-232)