The objective of this study was to determine whether Bordetella bronchiseptica would predispose to colonization or disease with Haemophilus parasuis. Three experiments were completed. In the first experiment, three groups of pigs (10 pigs/group) were inoculated intranasally with either B. bronchiseptica, H. parasuis, or with B. bronchiseptica followed by H. parasuis 1 week later. A fourth group of 10 pigs served as a non-infected control group. The second experiment was like the first, except that there were only five pigs per experimental group. The third experiment consisted of only two groups (10 pigs/group), one of which was inoculated intranasally with H. parasuis, whereas the other was inoculated with B. bronchiseptica followed by H. parasuis 1 week later. Pigs were necropsied 1-2 weeks after inoculation with H. parasuis. Mean nasal colonization by H. parasuis was significantly higher in the coinfected groups compared to the groups infected with H. parasuis alone. Pneumonia was present in 9/25 pigs coinfected with B. bronchiseptica and H. parasuis, 5/25 pigs infected with H. parasuis alone, 1/15 pigs infected with B. bronchiseptica alone, and in none of the pigs in the non-inoculated groups. Thus, B. bronchiseptica increased colonization of the upper respiratory tract with H. parasuis.