In pertussis-like respiratory infections, once pertussis has been laboratory confirmed, other potential causative pathogens will seldom be looked for. Probably most mixed infections are found accidentally and since these mixed infections might cause a more severe disease we performed a retrospective study of their incidence.We selected from 2 groups of patients with serologically confirmed Bordetella (B.) pertussis infection those in whom serology for other respiratory pathogens had also been performed. Group 1 consisted of 50 pertussis patients with 51 episodes of B. pertussis infection selected from 100 patients with serologically confirmed pertussis. They participated in a long-term follow-up after a B. pertussis infection. In group 2, 31 pertussis patients were selected from 98 consecutive patients with positive pertussis serology from one routine practice.In 23 of 82 pertussis infections (28%) serological evidence of 1 (n = 21) or 2 (n = 2) additional infections were demonstrated. These involved para-influenza virus (n = 6), respiratory syncytial virus (RSV) (n = 6), Mycoplasma pneumoniae (n = 5), adenovirus (n = 4), influenza A virus (n = 3) and influenza B virus (n = 1).We conclude that in patients with B. pertussis infection, coinfection with another respiratory pathogen is often present.