Background.Limited information is available on the cellular characteristics of the middle ear fluid (MEF) during acute otitis media (AOM).Objectives.To determine the white blood cell (WBC) composition of the MEF in AOM before and during antibiotic therapy.Materials and methods.Total WBC and differential counts were determined in the MEF of 96 infants and children (ages 2 weeks to 3 years) with AOM who were receiving antibiotics. WBC counts were reported as number of WBC/mg MEF (mean ± sd).Results.One hundred forty-five MEF samples were obtained by tympanocentesis at enrollment (Day 1), and 36 samples were collected on Days 4 to 5 after initiation of antibiotic therapy. Sixty-one percent of the patients were <1 year of age, and 38% were receiving antibiotic therapy at enrollment. Twenty-eight MEF samples were paired (same ear, Day 1 and Days 4 to 5). One hundred twelve pathogens were isolated from 95 of 145 (66%) culture-positive samples obtained on Day 1: 67Haemophilus influenzae, 40Streptococcus pneumoniaeand 5 others. MEF WBC counts were lower on Day 1 in patients who had received previous antibiotic therapy than in those who had not (432.4± 412.8 vs.590.5 ± 436.8,P= 0.03). WBC counts were higher on Day 1 in culture-positive than in culture-negative samples (603.9 ± 504.9 vs.421.4 ± 373.4,P= 0.02). WBC counts were higher on Day 1 in MEF samples positive forS. pneumoniaethan in those positive forH. influenzae(799.2 ± 641.5 vs.506.4 ± 401.9,P= 0.04). There were no differences in the number of neutrophil WBC present in the samples obtained on Day 1 vs.Days 4 to 5 or between samples positive vs.samples negative for bacterial pathogens.Conclusions.WBC counts were higher in the MEF of patients with culture-positive AOM than in those with culture-negative AOM and in those with AOM caused byS. pneumoniae.