A retrospective study was conducted among patients with dengue infection admitted to Rayong Hospital during September 2004-September 2005. Data were collected from medical charts and outpatient records created when the patients came to the hospital. Of the patients diagnosed with dengue, only 301 who met the WHO criteria for dengue fever and DHF/DSS were selected. The study cohort was comprised of 147 children (76 males, 71 females) and 154 adults (71 males, 83 females), with an overall mean age of 17.6 years. Some adult clinical symptoms were different from the children. Headache and myalgia were more common among adults (p < 0.05), but cough, vomiting, abdominal pain, and rash were more common among children (p < 0.05). Among the major bleeding symptoms, epistaxis (nasal bleeding) was more common in children (p = 0.012) and gum bleeding was more common in adults (p < 0.001). Myalgia was more likely in less severe grades of infection. Adults showed some different clinical manifestations of dengue infection from children. It is necessary for health personnel to take these differences into consideration when seeing probable cases of dengue infection.