Acute rhinosinusitis is an acute infection of the nasal passages and paranasal sinuses that lasts less than four weeks. Diagnosis of acute rhinosinusitis is generally based on clinical signs and symptoms in ambulatory care settings. Technical investigations are not routinely performed, nor are they recommended in most countries. Some trials show a trend in favour of antibiotics, but the balance of benefit versus harm is unclear.We merged two Cochrane Reviews for this update, which comprised different approaches with overlapping populations, resulting in different conclusions. For this review update, we maintained the distinction between populations diagnosed by clinical signs and symptoms, or imaging. To assess the effects of antibiotics versus placebo or no treatment in adults with acute rhinosinusitis in ambulatory care settings. We searched CENTRAL (2017, Issue 12), which contains the Cochrane Acute Respiratory Infections Group's Specialised Register, MEDLINE (January 1950 to January 2018), Embase (January 1974 to January 2018), and two trials registers (January 2018). We also checked references from identified trials, systematic reviews, and relevant guidelines. Randomised controlled trials of antibiotics versus placebo or no treatment in people with rhinosinusitis-like signs or symptoms or sinusitis confirmed by imaging. Two review authors independently extracted data about cure and side effects and assessed the risk of bias. We contacted trial authors for additional information as required. We included 15 trials involving 3057 participants. Of the 15 included trials, 10 appeared in our 2012 review, and five (631 participants) are legacy trials from merging two reviews. No new studies were included from searches for this update. Overall, risk of bias was low. Without antibiotics, 46% of participants with rhinosinusitis, whether or not confirmed by radiography, were cured after 1 week and 64% after 14 days. Antibiotics can shorten time to cure, but only 5 to 11 more people per 100 will be cured faster if they receive antibiotics instead of placebo or no treatment: clinical diagnosis (odds ratio (OR) 1.25, 95% confidence interval (CI) 1.02 to 1.54; number needed to treat for an additional beneficial outcome (NNTB) 19, 95% CI 10 to 205; I² = 0%; 8 trials; high-quality evidence) and diagnosis confirmed by radiography (OR 1.57, 95% CI 1.03 to 2.39; NNTB 10, 95% CI 5 to 136; I² = 0%; 3 trials; moderate-quality evidence). Cure rates with antibiotics were higher when a fluid level or total opacification in any sinus was found on computed tomography (OR 4.89, 95% CI 1.75 to 13.72; NNTB 4, 95% CI 2 to 15; 1 trial; moderate-quality evidence). Purulent secretion resolved faster with antibiotics (OR 1.58, 95% CI 1.13 to 2.22; NNTB 10, 95% CI 6 to 35; I² = 0%; 3 trials; high-quality evidence). However, 13 more people experienced side effects with antibiotics compared to placebo or no treatment (OR 2.21, 95% CI 1.74 to 2.82; number needed to treat for an additional harmful outcome (NNTH) 8, 95% CI 6 to 12; I² = 16%; 10 trials; high-quality evidence). Five fewer people per 100 will experience clinical failure if they receive antibiotics instead of placebo or no treatment (Peto OR 0.48, 95% CI 0.36 to 0.63; NNTH 19, 95% CI 15 to 27; I² = 21%; 12 trials; high-quality evidence). A disease-related complication (brain abscess) occurred in one participant (of 3057) one week after receiving open antibiotic therapy (clinical failure, control group). The potential benefit of antibiotics to treat acute rhinosinusitis diagnosed either clinically (low risk of bias, high-quality evidence) or confirmed by imaging (low to unclear risk of bias, moderate-quality evidence) is marginal and needs to be seen in the context of the risk of adverse effects. Considering antibiotic resistance, and the very low incidence of serious complications, we conclude there is no place for antibiotics for people with uncomplicated acute rhinosinusitis. We could not draw conclusions about children, people with suppressed immune systems, and those with severe sinusitis, because these populations were not included in the available trials.