A-mode ultrasound examination of the maxillary sinuses with the Echosine and Sinusvu 2500 units was compared with roentgenographic examination in the diagnosis of maxillary and frontal sinusitis. A-mode ultrasound was primarily useful in the detection of secretions within the sinus and not mucosal thickening. Both ultrasound devices had high specificities (92% for Echosine and 94% for Sinusvu 2500) in the diagnosis of maxillary sinusitis, but the Echosine had a greater overall sensitivity (61%) than the Sinusvu 2500 (29%). More significantly, as the opacification of the roentgenogram increased (a higher probability of secretion), the Echosine demonstrated better agreement with the x-ray film. Ultrasound appeared to be less helpful in diagnosing frontal sinusitis, but there were too few cases of frontal sinusitis for statistical analysis. The Echosine is a useful screening device because of the high specificity of a positive result. Although A-mode ultrasound provides limited value in diagnosing mucosal thickening, it is particularly useful in following the course of therapy once a positive diagnosis of sinusitis has been established without subjecting the patient to additional x-ray exposure.