An acute pneumonia developed in 28 calves which had been housed together from one to two weeks of age. The clinical signs included pyrexia, tachypnoea, respiratory distress and coughing. Some of the calves died. The pneumonia was characterised by an alveolitis with multinucleated syncytia, alveolar epithelial hyperplasia and bronchiolitis. Interstitial emphysema was also present. Fifteen of 19 calves examined serologically had rising neutralising antibody titres to respiratory syncytial virus; in nine calves the rise was fourfold or greater. Respiratory syncytial virus was not isolated from the calves. There was no evidence of parainfluenza type 3 virus involvement. The adult cows being sucked by the calves remained clinically normal throughout the incident. Six calves examined six weeks after the outbreak started had a chronic cuffing pneumonia characterised by lymphocytic bronchiolitis; some of the calves also had bronchiolitis obliterans. Mycoplasma dispar was found in two of them.