OBJECTIVE--To evaluate the diagnostic value of symptoms, signs, erythrocyte sedimentation rate, and C reactive protein for acute maxillary sinusitis. DESIGN--Prospective cohort study. SETTING--Danish general practice in cooperation with the otorhinolaryngology and neuroradiology department at Aalborg County Hospital. SUBJECTS--174 patients aged 18-65 years who were suspected by the general practitioner of having acute maxillary sinusitis. MAIN OUTCOME MEASURE--The independent association of symptoms, signs, erythrocyte sedimentation rate, and concentration of C reactive protein in patients with acute maxillary sinusitis defined as purulent or mucopurulent antral aspirate. RESULTS--Only raised erythrocyte sedimentation rate (P = 0.01) and raised C reactive protein (P = 0.007) were found to be independently associated with a diagnosis of acute maxillary sinusitis. The combination of the two variables had a sensitivity of 0.82 and a specificity of 0.57. CONCLUSION--Erythrocyte sedimentation rate and C reactive protein are useful diagnostic criteria for acute maxillary sinusitis.