ABSTRACT. Objective. The spreading of acute respiratory infection into the paranasal sinuses in children is poorly defined. The main objective of this study was to evaluate the frequency and spontaneous resolution of paranasal sinus abnormalities in children with acute respiratory infection using magnetic resonance imaging (MRI).Methods. We examined 60 children with MRI (mean age: 5.7 years) with symptoms of acute respiratory infection. Twenty-six children with major abnormalities in the first MRI scan had a follow-up MRI taken 2 weeks later.Results. The children had had symptoms of uncomplicated acute respiratory infection for an average of 6 days before the first examination (mean duration: 6. Conclusions. These observations indicate that acute respiratory infection mostly spreads into the paranasal sinuses of children in the form of mucosal edema and that these abnormalities tend to resolve spontaneously without antimicrobial treatment. Pediatrics 2003;111:e586 -e589. URL: http://www.pediatrics.org/cgi/content/full/111/5/e586; sinusitis, children, acute respiratory infection, MRI.