Lung ultrasound (LUS) is a more sensitive method of detecting pathological pulmonary changes than chest X-ray. Therefore, LUS for patients with dengue could be an important tool for the early detection of pleural effusions and pulmonary edema signifying capillary plasma leakage, which is the hallmark of severe dengue pathophysiology. We conducted a prospective observational study of pulmonary changes identifiable with LUS in dengue patients admitted to the Hospital for Tropical Diseases in Mahidol University, Bangkok, and the Bamrasnaradura Infectious Diseases Institute, Nonthaburi, Thailand. The LUS findings were described according to standard criteria, including the presence of A, B1, B2, and C patterns in eight chest regions and the presence of pleural effusions. From November 2017 to April 2018, 50 patients with dengue were included in the study. LUS was performed during the febrile phase for nine patients (18%) and during the critical-convalescence phase for 41 patients (82%). A total of 33 patients (66%) had at least one abnormality discovered using LUS. Abnormal LUS findings were observed more frequently during the critical-convalescence phase (N = 30/41; 73%) than during the febrile phase (N = 3/9; 33%) (P = 0.047). Abnormal aeration patterns were observed in 31 patients (62%). Only B patterns with only multiple B lines were observed in 21 patients (42%); of these patients, three had already exhibited B patterns during the febrile phase (N = 3). C patterns (N = 10; 24%), pleural effusion (N = 10; 24%), and subpleural abnormalities (N = 11; 27%) were observed only during the critical-convalescence phase. LUS can detect signs of capillary leakage, including interstitial edema and pleural effusions, early during the course of dengue.