In primary care, acute maxillary sinusitis may be diagnosed by clinical examination, ultrasound or radiography. Previous studies on the diagnostic accuracy of these methods are from secondary care settings and may not be generalisable to primary care. In this study of 39 primary care patients we have compared ultrasound, clinical examination and radiography to sinus irrigation. The sensitivity of ultrasound performed by general practitioners is 61% and specificity is 53%. Diagnostic accuracy does not improve when the general practitioner bases the diagnosis on combination of clinical examination and ultrasound. The most accurate way to diagnose sinusitis is radiography and when the radiographs are interpreted by a radiologist (sensitivity: 61%; specificity: 98%). The accuracy of the ultrasound examination performed by general practitioners is poorer than earlier results from ENT practices. More attention should be paid to education and quality management in the use of ultrasound in primary care.