In primary care, the diagnosis of acute maxillary sinusitis (AMS) is based most often only on symptoms and clinical examination. This practice leads to many false positive diagnoses and unnecessary use of antibiotics. Sinus ultrasound has been suggested as a means to improve the accuracy of AMS diagnosis. We studied the symptoms, signs, and ultrasound findings in patients with acute rhinosinusitis. Primary care physicians received small group teaching on sinus ultrasound technique. Sinus radiography was performed in a subgroup of patients, and it acted as reference standard. A total of 150 adult patients were recruited to the study, 105 women (70%) and 45 men (30%). Thirteen patients of 32 (41%) in the radiography subgroup and 74 patients of 148 (50%) with ultrasound result had AMS. The sensitivity of ultrasound compared to radiography was 92% and specificity was 95% when results were calculated per patients as unit of analysis. With practice and teaching primary care physicians can perform sinus ultrasound as accurately as specialists. Symptoms and clinical examination were not reliable in AMS diagnosis. If the criterion for AMS diagnosis were fluid in maxillary sinuses in ultrasound instead of clinical impression, the number of antibiotic prescriptions would be reduced by half in primary care.