In a prospective clinical study, rates of isolation of Moraxella catarrhalis in nasopharyngeal aspirates from 122 children with respiratory tract infection and 72 healthy controls were compared. In the patient group, Moraxella catarrhalis and Streptococcus pneumoniae were the most frequently isolated pathogens (38% and 42%, respectively). Monocultures of each pathogen were equally distributed in patients and controls (41% and 42%), whereas mixed infections were found more frequently in the patient group (42% vs 14%; normal flora, 17% vs 44%). Moraxella catarrhalis appears to be a relevant respiratory pathogen. The isolation of two or more pathogens in nasopharyngeal aspirates seems to be as indicative of relevant infection as is monoculture.