Sinusitis is a common disease with harmful effects on the health and finances of patients and the economy of the community. It is easily treated in most of its acute stages but is associated with some management difficulties as it goes toward chronicity. Therefore, we tried to improve the treatment of subacute sinusitis by using acetylcysteine, which is a safe mucolytic and antioxidant agent. Thirty-nine adult patients with subacute sinusitis proved by computed tomography (CT) were enrolled in a double-blind, placebo-controlled trial. They received oral amoxicillin-clavulanic acid and normal saline nasal drops for 10 days and oral pseudoephedrine for 7 days. In addition, the patients received acetylcysteine (600 mg orally, once daily) in the intervention group or placebo in the control group for 10 days. A paranasal CT scan was taken at baseline and 30 days after patients finished the treatment and was evaluated quantitatively by Lund-Mackay (LM) score. Symptoms and some aspects of quality of life also were assessed at baseline and 14 days after initiation and 30 days after termination of the treatment via the Sino-Nasal Outcome Test questionnaire. The groups showed no significant difference in LM score after treatment. A positive correlation was observed between the LM and SNOT-20 scores. We concluded that adding oral acetylcysteine to amoxicillin-clavulanic acid, pseudoephedrine, and intranasal normal saline has no benefit for the treatment of subacute sinusitis.