The aim of the present study was to evaluate whether the results of such simple hematologic tests as erythrocyte sedimentation rate (ESR), white blood cell count (WBC) and C-reactive protein (CRP) could give any useful information about the causative agents in 176 patients with acute maxillary sinusitis (AMS). The great majority of tests (82%) showed values which were within normal limits. This may be due to the fact that 22% of AMS cases were culture negative for bacteria and about 60% of culture positive cases had Haemophilus influenzae as the etiologic agent. Significantly raised test values were seen in connection with Streptococcus pyogenes in the majority of cases, less frequently with Streptococcus pneumoniae and rarely with Haemophilus influenzae. We conclude that none of the evaluated routine blood tests are particularly sensitive indicators of the specific etiology of AMS in general. However, elevated CRP values (> 40 mg/l) associated with AMS should alert the physician to the suspicion of Streptococcus pyogenes or Streptococcus pneumoniae in etiology, since both, if left untreated, may lead to sinus empyema. With CRP or other evaluated tests Haemophilus influenzae- or Branhamella catarrhalis-positive AMS cannot be distinguished from a purely viral disease.