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In order to dissect the histopathological changes produced by the infection of avian infectious bronchitis virus in previously vaccinated egg-laying hens from a poultry farming unit, 35 White Leghorn egg-laying hens that had been in production for 9 to 10 months (twenty seven of which had clinical symptoms corresponding to respiratory disease and eight apparently healthy individuals) were selected for further study. After clinical examination and necropsy, they were classified into apparently healthy, mild, moderate or severe according to the severity of the clinical-pathological process. Samples were taken from paranasal sinuses, trachea and lungs for histopathological study, and trachealung pools were prepared from four individuals for virus isolation and molecular biology assays. The presence of mucus was evidenced with Schiff's non-enzymatic histochemical staining, and histomorphometric analyses were used to estimate the number of glands in the tracheal mucosa. The proportions of histopathological lesions were compared, using one-way Anova to determine gland loss at the tracheae with a signifi cance level of p < 0.05 in both cases. Histopathological analysis of the epithelia of paranasal sinuses, trachea and bronchia revealed the presence of epithelial erosion, mucous exudate and hyperplasia of mucosa-associated lymphoid tissue. Glandular cysts were observed at the paranasal sinuses, and epithelial metaplasia was detected in the trachea. It was possible to isolate and identify infectious bronchitis coronavirus from the original samples and from samples passaged in chicken embryos. [ABSTRACT FROM AUTHOR]