Background: Sinusitis is a common disease in children. Even though new research findings are continually being published on various aspects of this disease, reports of its clinical characteristics and associated conditions are still limited. Objective: To describe the characteristics and associated conditions of acute sinusitis in Thai children. Materials and Methods: Our study consisted of 140 acute sinusitis patients who presented to Thammasat University Hospital. Acute sinusitis is diagnosed on the basis of clinical presentation including persistent, severe or worsening nasal symptoms. An informed consent form was obtained from each patient before the study. Radiography has been used to aid in the diagnosis of acute sinusitis. Allergic skin prick test was also performed in this study. Results: Age range of the 140 patients were between 1.1 to 15 years with a mean (+/-SD) of 5.67 +/-2.85 years. History of allergic rhinitis, confirmed by positive skin prick test or nasal cytology, was found in 64%. The parental history of atopy was found in 50.4 %. Positive history of tobacco smoking was found in 25.9 %. Familial members with diagnosis of a cold or sinus infection during the same time was found in 43.2 %. 69.8% of the patients attended day care or school more than five days per week. 38.1% of the patients had a history of swimming or diving during the past 1 month. 51.8 % of the patients had a past history of sinus disease. The four most common symptoms were nasal blockage (88.5%), rhinorrhea (81.3%), nighttime cough (79.9%) and snoring (73.4%). The three most common signs were swelling of turbinates (75.5%), erythema of turbinates (74.8%) and tonsillar enlagement (48.2%). All paranasal sinuses X-rays were abnormal with maxillary sinus being the most commonly involved sinus (97.5%) followed by ethmoid sinus (58.8%). 58.8% of the patients had involvement of more than one sinus. Adenoid hypertrophy was positive in 36 % of all patients. The skin prick tests were positive in 62.6 % of all patients who the tests were performed. Conclusion: Nasal blockage, rhinorrhea, nighttime cough and snoring are very common in acute sinusitis. The present study demonstrates that allergic rhinitis and recurrent sinusitis is common in our patients.