Tympanometry was performed in 151 healthy children at birth and at 3 and 6 months of age. At birth 10.5% of the ears had a pressure of -100 mmH2O, and 0.3% a pressure of -125 mmH20. The pressure in most of these ears later became normalized. At 3 months of age 17.9% had a pressure of -100 mmH2O or less, and at 6 months of age 39.2%. At 6 months of age 1.3% of the ears had flat curves and middle ear effusion, and 9.6% had a pressure between -200 and -300 mmH2O. The pressure changes and their cause were analysed. Catarrhalia occurred in 23% of the infants before 3 months of age and in 60% in the 3-6 month period. This is an important though not the only etiological factor for the reduced ventilation in the middle ear, which probably occurs via oedema of the tubal mucosa and mild internal tubal occlusion. Ventilation was significantly reduced at 3 months of age in catarrhal girls, and in catarrhal boys at 6 months of age.