To determine the magnitude of the problem posed by primary dengue infection in children and the distinctive clinical clues that may differ from those with secondary infection, 996 children serologically diagnosed with dengue infection and admitted to the Department of Pediatrics, Chulalongkorn Hospital, Bangkok, Thailand between 1988 and 1995 were retrospectively reviewed. One hundred and thirty-nine cases (14.0%) were serologically proved to be primary dengue infection. Of these, 72 were males and 67 were females, with a mean age of 4.8 years. Common manifestations by order of frequency included fever (97.8%), hepatomegaly (71.9%), vomiting (59.0%), decreased appetite (55.4%), coryza (52.5%), drowsiness (39.6%), diarrhea (34.5%), rash (33.8%), abdominal pain (23.0%) and seizure (15.8%). The mean duration of fever before admission was 4.6 days. Common sites of bleeding were skin (41.7%), mucous membrane (14.4%) and the gastrointestinal tract (12.2%). Clinical diagnosis was categorized into dengue fever (22.3%), dengue hemorrhagic fever (60.4%) and dengue shock syndrome (17.3%). Three patients (2.2%) died. Compared with the children with secondary dengue infection (n=139), children with primary dengue infections tended to be younger, presented more commonly with coryza, diarrhea, rash and seizure; and less commonly with vomiting, headache and abdominal pain (p < 0.05). The maximal hematocrit level, the mean difference between maximal and minimal hematocrit values and the maximal percentage of neutrophils were significantly lower in the study group, whereas the maximal percentage of lymphocytes was significantly higher. Dengue fever was more common and dengue shock syndrome was less common in the study group (p < 0.05). This study has emphasized that primary dengue infection is not uncommon and is less severe than secondary infection. Clinical presentations and laboratory findings are somewhat different between the two conditions.