The objective of this study was to assess symptoms and signs in patients with maxillary sinusitis and a bacteriological diagnosis obtained by sinus aspiration or lavage. Designed as a prospective cohort study in general practice, the study included 174 patients, aged 18-65 years, suspected of having acute maxillary sinusitis by their general practitioner. The main outcome measures were the independent association of symptoms, signs, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) concentration and confirmed infection with the predominant bacterial pathogens Streptococcus pneumoniae and Haemophilus influenzae. The predominant organisms found in patients with acute maxillary sinusitis were S. pneumoniae and H. influenzae. Body temperature >38 degrees C and maxillary toothache were significantly associated with the presence of S. pneumoniae and H. influenzae. Positive bacteriological culture results were significantly associated with increasing ESR and CRP values. None of the symptoms and signs, with the exception of body temperature >38 degrees C and maxillary toothache, were particularly sensitive indicators of the specific aetiology in patients with acute maxillary sinusitis. Elevated ESR and CRP values were significantly associated with positive bacteriological culture results. On the other hand, absence of these symptoms and signs did not exclude the presence of acute maxillary sinusitis.