OBJECTIVE: To evaluate the effect of C-reactive protein (CRP) testing on the antibiotic prescribing in patients with acute rhinosinusitis. METHODS: Audit-based study carried out in primary care centres in Spain. GPs registered episodes of rhinosinusitis during 3-week period before and after an intervention. Two types of intervention were considered: full intervention group (FIG) consisting in individual feedback based on results from the first registry, courses in rational antibiotic prescribing, guidelines, patient information leaflets, workshops on rapid tests and use of the CRP test. GPs in the partial intervention group (PIG) underwent all the above intervention except for the workshop and they did not have access to CRP. Multilevel logistic regression analysis was performed considering the prescription of antibiotics as the dependent variable. RESULTS: Two hundred and ten physicians were assigned to FIG and 71 to PIG. In 2009, 59 new physicians were included as a control group. Two hundred and sixty-seven GPs visited contacts with rhinosinusitis (78.5%) registering a total of 836 cases. In the group of GPs with access to CRP rapid test, 207 patients with rhinosinusitis (75.3%) were tested and antibiotics were prescribed in 156 patients (56.7%). Antibiotics were prescribed in 87 patients (82.9%) in the group of GPs exposed to PIG and in 52 patients (86.7%) in the control group (P <0.001). Antibiotic prescription was significantly reduced after the intervention among physicians assigned to FIG, with an odds ratio of antibiotic prescribing of 0.12 (95% confidence interval: 0.01-0.32). CONCLUSION: Physicians with access to CRP tests significantly reduced antibiotic prescription in patients with rhinosinusitis.