Acute sinusitis is the inflammation and swelling of the nasal and paranasal mucous membranes and is a common reason for patients to seek primary care consultations. The related impairment of daily functioning and quality of life is attributable to symptoms such as facial pain and nasal congestion. To assess the effects of systemic corticosteroids on clinical response rates and to determine adverse effects and relapse rates of systemic corticosteroids compared to placebo or standard clinical care in children and adults with acute sinusitis. We searched CENTRAL (2014, Issue 1), MEDLINE (1966 to February week 1, 2014) and EMBASE (January 2009 to February 2014). Randomised controlled trials (RCTs) comparing systemic corticosteroids to placebo or standard clinical care for patients with acute sinusitis. Two review authors independently assessed the methodological quality of the trials and extracted data. Five RCTs with a total of 1193 adult participants met our inclusion criteria. We judged methodological quality to be moderate in four trials and high in one trial. Acute sinusitis was defined clinically in all trials. However, the three trials performed in ear, nose and throat (ENT) outpatient clinics also used radiological assessment as part of their inclusion criteria. All participants were assigned to either oral corticosteroids (prednisone 24 mg to 80 mg daily or betamethasone 1 mg daily) or the control treatment (placebo in four trials and non-steroidal anti-inflammatory drugs (NSAIDs) in one trial). In four trials antibiotics were prescribed in addition to oral corticosteroids or control treatment, while one trial investigated the effects of oral corticosteroids as a monotherapy.When combining data from the five trials, participants treated with oral corticosteroids were more likely to have short-term resolution or improvement of symptoms than those receiving the control treatment: at days three to seven (risk ratio (RR) 1.3, 95% confidence interval (CI) 1.1 to 1.6; risk difference (RD) 17%, 95% CI 6% to 29%) and at days four to 14 (RR 1.2, 95% CI 1.0 to 1.5; RD 14%, 95% CI 1% to 27%). A sensitivity analysis including the four trials with placebo as a control treatment showed similar results but with a lesser effect size: at days three to seven (RR 1.2, 95% CI 1.1 to 1.3; RD 11%, 95% CI 4% to 17%) and days four to 14 (RR 1.1, 95% CI 1.0 to 1.2; RD 8%, 95% CI 2% to 13%). Statistical heterogeneity was high for many analyses. Subgroup analyses revealed that corticosteroid monotherapy had no beneficial effects. Furthermore, scenario analysis showed that outcomes missing from the trial reports might have introduced attrition bias (a worst-case scenario showed no statistically significant beneficial effect of oral corticosteroids). No trial reported effects on relapse or recurrence rates. Reported side effects in patients treated with oral corticosteroids were mild (nausea, vomiting, gastric complaints) and did not significantly differ from those receiving placebo. Oral corticosteroids as a monotherapy appear to be ineffective for adult patients with clinically diagnosed acute sinusitis. Current data on the use of oral corticosteroids as an adjunctive therapy to oral antibiotics are limited: almost all trials are performed in secondary care settings and there is a significant risk of bias. This limited evidence suggests that oral corticosteroids in combination with antibiotics may be modestly beneficial for short-term relief of symptoms in acute sinusitis, with a number needed to treat to benefit of seven for resolution or symptom improvement. A large primary care factorial trial is needed to establish whether oral corticosteroids offer additional benefits over antibiotics in acute sinusitis.