A blinded, randomised clinical trial was carried out in Brittany, France on three commercial pig farms with a history of pneumonia. Pigs with clinical signs of respiratory disease were randomly allocated to one of two treatment groups; 100 pigs received a single intramuscular injection of a long-acting formulation of tylosin at a dose rate of 20 mg tylosin/kg bodyweight, and 101 pigs received three consecutive daily intramuscular injections of 10 mg tylosin/kg bodyweight. The pigs' rectal temperatures and other clinical variables were recorded at intervals and a scoring system was used to evaluate the results of the treatments. Relapses were recorded for up to nine days after the treatment. There were no statistically significant differences between the two treatments in terms of clinical scores, rectal temperatures, or cure or relapse rates.