Acute sinusitis is often a mild, self-limiting disease. However, in some cases, especially among children, sinusitis may become a severe, even life-threatening, disease. To examine the nature of complications of acute sinusitis, we studied the cases of children treated at the Helsinki University ENT Hospital, because of a complication caused by acute sinusitis from January 1997 to September 1998. There were 12 children (4 girls, 8 boys), whose ages ranged from 16 months to 16 years. One child had an epidural abscess, one got meningitis and cavernous sinus thrombosis, five had orbital cellulitis, one of whom lost her vision permanently in one eye, and five had preseptal cellulitis. All the children were treated with intravenous antibiotics and all, except the youngest, were treated with a direct sinus puncture. An operation (intranasal antrostomy, orbital drainage, functional endoscopic sinus surgery or adenoidectomy) was performed on six patients. In the majority of children, acute sinusitis is a mild self-limiting disease. However, severe complications still exist. When a complication of sinusitis is suspected, it is of utmost importance that the child be sent immediately to a hospital for proper diagnosis and treatment.