SUMMARY There is no final proof of the mechanism of how farmer's lung is caused. Clinical observation of patients leaves no doubt that it is an entity. The mode of onset is typical, the symptoms show a remarkable uniformity and the x-ray appearances are characteristic. The histology of all the cases published shows a close uniformity. The relatively slow appearance of this granulomatous change accounts for the fact that x-ray changes are not apparent in the first three to four weeks of the condition. After hay dust has been fractionated it is only the coarser particles which appear to contain the causative factor. Thus, farmer's lung may be regarded as a granulomatous infiltration of the lungs, which occurs in agricultural and other workers, who have become sensitized to hay or threshing dust. The condition can, if recurrent, cause permanent damage to the lungs in the form of fibrosis and bronchiectasis. It bears no relationship to asthma. The treatment is first and foremost the avoidance of the cause. A number of patients in the group which occurred in Devon have been able to continue their farm work provided they wear efficient masks. Others have been forced to change their occupation. Prednisone has been used for short periods in a few cases with apparent success.