Retained secretion within the maxillary sinus was diagnosed by ultrasonography in 86 (19%) of 452 children (4-10 years) who entered Turku University ENT-clinic for adenoidectomy or adenotonsillectomy. Occipitomental radiographs showed signs of sinusitis in 168 (37%) of the children. All 86 children with secretion in one or both maxillary sinuses were operated on and treated with amoxicillin and a nasal decongestant. Fifty children (72 sinuses) were treated with antral lavage and 36 children (49 sinuses) were treated conservatively. After 10 days ultrasonography revealed no secretion in 86% of the irrigated sinuses and in 74% of the sinuses treated without irrigation. After 30 days all the irrigated sinuses were healed but in 5 (10%) of the sinuses treated without irrigation ultrasonography still revealed retained secretion. The difference was statistically significant. The disappearance of the radiographic sinus changes was considerably slower. As a non-ionizing method ultrasonography was found to be especially practical in the follow-up of treatment results. It seems that the disappearance of ultrasonic signs of sinusitis correlates better with the clinical resolving of sinusitis in childhood than the disappearance of radiographic signs of sinusitis.