The bacteriology of middle ear effusion (MEE) of asymptomatic otitis media with effusion (OME) was studied in 165 children, aged 5 months to 12 years, from the MEE samples obtained during tympanostomy under general anaesthesia in 1993-1994. MEE had persisted for 1 to 12 (mean 3.5) months. Major otitis pathogens (S. pneumoniae, H. Influenzae, B. catarrhalis and S. pyogenes) were cultured in 41% of the children under 2 years of age and in 17% of older children (p < 0.001). Respiratory infections and attacks of acute otitis media (AOM) during the last 6 months were also more frequent in children younger than 2 years (p < 0.001). The proportion of S. pneumoniae (25%) and H. influenzae (38%), but not of other bacteria, was higher in the children with less than 2 months' persistence of MEE as compared with those with a longer duration (8% and 3%) (p < 0.01). After 2 months, the occurrence of different bacteria remained relatively unchanged until 6 months' persistence of MEE, and thereafter no pathogens were culturable. Among the children adenotomized earlier, the proportion of those with major otitis pathogens in MEE was 8% compared with 32% in non-adenotomized children (p = 0.02). S. pneumoniae, B. catarrhalis or S. pyogenes were not culturable in any of the adenotomized children, while MEE grew them in 25% of the non-adenotomized children (p < 0.001). Since the MEE bacteriology of OME with less than 2 months' persistence resembles that of AOM, it may be that these cases represent a transitory phase between AOM and an established OME.