An epidemiological study of atrophic rhinitis was carried out in four pig herds. Observations were made of (i) infection with Bordetella bronchiseptica and Pasteurella multocida, (ii) the presence of brachygnathia superior (BS score), (iii) the extent (grade) of turbinate atrophy and pneumonia at slaughter and (iv) growth rates from two to 16 weeks of age and average daily weight gains to slaughter. In two of the herds with no history of atrophic rhinitis, B bronchiseptica and non-toxigenic strains of P multocida were isolated; only one of 47 pigs (2 per cent) had a BS score greater than +10 mm and the most severe turbinate atrophy observed in 21 pigs at slaughter was grade 3. In contrast, from two herds with atrophic rhinitis, toxigenic strains of P multocida were isolated as well as B bronchiseptica and non-toxigenic P multocida. BS scores of greater than +10 mm were present in six of 47 pigs (13 per cent) of which five were infected with toxigenic P multocida and had severe turbinate atrophy of grade 4 or 5. There was no significant reduction in growth rates in the affected compared with the unaffected herds nor in the affected compared with the unaffected pigs in the same herd. Neither was there a correlation between progressive disease and the extent of pneumonia found at slaughter. It was concluded that in field cases of the disease, high BS scores plus severe turbinate atrophy were associated with infection by toxigenic type-D strains of P multocida.