The clinical aspects as well as the pathology, microbiology and serology of a natural Bovine Respiratory Syncytial (BRSV) infection of bovine in a herd of 600 beef cattle kept under extensive management in the State of Rio Grande do Sul, Brazil, are described. Clinically two animals had chronic cough and severe dyspnea when forced to mild physical exercise. These two animals were euthanatized and post-morten examination was performed. The macroscopic changes were of pulmonary origin, such as disseminated alveolar emphysema, focal atelectasis and marked interlobular septal thickening. The fluorescent antibody test on lung cryostat sections was positive to BRSV for both animals, and it was negative to PI-3 virus, BVDV and BHV. The BRSV was isolated from the lung of one of the animals on MDBK, and was also identified by fluorescent antibody test. No association with Chlamydia psittaci was found by ELISA performed on lung tissues. The histopathology showed syncytial cells, chronic emphysema, peribronchiolar muscle layer hypertrophy and squamous metaplasia of bronchial and bronchiolar epithelia. The serology to detect antibodies to BRSV resulted in 79% of positives from the first specimen collection. In this group of young animals some of them had a cough. The second samples collected 6 months later were from animals of different age groups resulting in 17.3% of positives. This is the first report on clinical BRSV infection in Brazil.1727781Bryson, D.G., McNulty, M.S., Logan, E.F., Cush, P.F., Respiratory syncytial virus pneumonia in young calves. Clinical and pathologic findings (1983) Am. J. Vet. Res., 44, pp. 1648-1655Bryson, D.G., McFerran, J.B., Ball, H.L., Neill, S.D., Observations on outbreaks of respiratory disease in calves associated with parainfluenza type 3 virus and respiratory syncytial virus infection (1979) Vet Rec., 104, pp. 45-49Campalans, J.B., Arns, C.W., Isolation of Bovine Respiratory Syncytial Virus in Brazil (1995) Anais 5° Virológica 95, pp. B-34. , Ribeirão Preto, SPCarter, C.R., Cole, J.R., (1990) Diagnostic Procedures in Veterinary Bacteriology and Mycology. 5th Ed., 620p. , Academic Press, New YorkDungworth, D.L., The respiratory system (1993) Pathology of Domestic Animals, 2, pp. 539-699. , Jubb K.V.F., Kennedy P.C., Palmer N. (ed.) 4th ed. Academic Press, San DiegoEb, F., Orfila, J., Rapid direct immunological procedures (1988) Proc. European Society for Chlamydia Research, pp. 223-226. , Almqvist & Wiksell International, Bolonha, ItalyGonçalves, I.P.D., Simanke, A.T., Jost, H.C., Hötzel, I., Dal Soglio, A., Moojen, V., Detection of bovine respiratory syncytial virus in calves of Rio Grande do Sul, Brazil (1993) Ciência Rural, 23 (3), pp. 389-390. , Santa MariaGrand, M., Baxter-Jones, C., Wilding, G.P., An enzime-linked immunosorbent assay for the serodiagnosis of turkey rhinotracheitis infection (1987) Vet. Rec., 21, pp. 279-280Inaba, Y., Tonaka, I., Omori, T., Matumoto, M., Isolation of bovine respiratory syncytial virus (1970) Japan J. Exp. Med., 40, pp. 473-474Jacobs, J.W., Edington, N., Experimental infection of calves with respiratory syncytial virus (1975) Res. Vet. Sci., 18, pp. 299-306Kimman, T.H., Terpstra, G.K., Daha, M.R., Westenbrik, F., Pathogenesis of naturally acquired bovine respiratory syncytial virus infection in calves: Evidence for the involvement of complement and mast cell mediators (1989) Am. J. Vet. Res., 50, pp. 694-700Lehmkuhl, H.D., Cutlip, R.C., Experimental respiratory syncytial virus infection in feeder-age Iambs (1979) Am. J. Vet. Res., 40, pp. 1729-1730Paccaud, M.F., Jacquier, C., A respiratory syncytial virus of bovine origin (1970) Arch. Gesamte Virusforschung, 30, pp. 327-342Pirie, H.M., Petrie, L., Pringle, C.R., Allan, E.M., Kennedy, G.J., Acute fatal pneumonia in calves due to respiratory syncytial virus (1981) Vet. Rec., 108, pp. 411-416Prophet, E.B., Mills, B., Arrington, J.B., Sobin, L.H., (1992) Laboratory Methods in Histotechnology, 278p. , Armed Forces Institut of Pathology, Washington D.CRadostits, O.M., Blood, D.C., Gay, C.C., (1994) Veterinary Medicine. 8th Ed., 1763p. , Baillère Tindall, LondonStewart, R.S., Gershwin, L.J., Role of IgE in the pathogenesis of bovine respiratory syncytial virus in sequencial infections in vaccinated and nonvaccinated calves (1989) Am. J. Vet. Res., 50, pp. 349-354Van Der Ingh, T.S.G.A.M., Verhoeff, J., Van Nieuwstadt, A.P.K.M.I., Clinical and pathological observations on spontaneous bovine respiratory syncytial infections in calves (1982) Res. Vet. Sci, 33, pp. 152-158