Sinusitis is associated with overuse of antibiotics. The aim of this study was to determine whether GPs who use the CRP rapid test (CRP) have a lower antibiotic prescribing rate for sinusitis. During a three-week period, a group of GPs registered all patients with respiratory tract infections (n = 17,792). GPs using CRP prescribed antibiotics for 59% and GPs not using CRP prescribed antibiotics for 78% of the patients with sinusitis. CRP was the factor exerting the greatest influence on the prescribing of antibiotics. Implementing CRP in general practice may lead to a reduction in antibiotic prescribing to patients with sinusitis.