The incidence of adrenergic-induced arrhythmias was studied in 19 patients with chronic obstructive lung disease uncomplicated by respiratory failure or cardiac desease. The effects of isoproterenol hydrochloride and a placebo were studied, using a randomized double blind technique and continuous monitoring of the cardiac rhythm. Two of the patients had episodes of arrhythmia associated only with nebulization of isoproterenol hydrochloride. Ten patients had no abnormality associated with isoproterenol hydrochloride or placebo nebulizations. In the remainder, episodes of arrhythmia occurred but could not be consistently correlated with placebo or drug nrbulizations. None of the patients was aware of any cardiac irregularity. A chi square analysis of the dataindicated no relationship between the isoproterenol nebulizations and cardiac arrhythmias. In our patients with uncomplicated chronic obstructive lung disease, nebulization of adrenergic bronchodilators seemed an infrequent cause of cardiac arrhythmias.