One hundred and twenty-one visits of 58 infants (2-11 months of age) were evaluated in the Finnish Otitis Media Vaccine Trial. Infants were examined with tympanometry (Grason-Stadler GSI 38 Autotymp) and pneumatic otoscopy by one study doctor. Diagnosis of otitis media was verified by myringotomy in 74% of cases. Tympanometry was technically successful in 94% of ears. The success rate was statistically significantly higher (P < 0.05) among infants less than 7 months of age than those above 7 months. The sensitivity of tympanometry (type B) to detect ears with middle ear fluid was 0.70 and the specificity 0.98 with a positive predictive value of 0.93 and negative predictive value of 0.94. The sensitivity was somewhat lower in the younger age group (0.61); specificity and positive and negative predictive values were good in both age groups. The high success rate and high negative and positive predictive values of tympanometry make it a useful aid for assuring the correct diagnosis of otitis media in infants in routine clinical practice.