Accumulation of purulent sinus secretion reflects an inflammation caused by pyogenic bacteria, whereas the accumulation of nonpurulent effusion may have origins other than a bacterial infection. Ultrasonography differentiates the disordered sinus from the healthy one, but not purulent from nonpurulent secretion. In 90 patients, supposed to suffer from sinusitis, a correct diagnosis, sinus empyema vs. not sinus empyema, was established in a majority of cases by means of clinical evaluation alone. By introducing ultrasonography as a complement to the clinical evaluation, the diagnostic reliability became lower. It is concluded that ultrasonography seems to have little or no implication in the therapeutic decision unless diagnostic puncture is also performed.