During a 5-year period, 1969-1973, 451 patients with acute severe pulmonary diseases were admitted to the three medical intensive care units (MICU) at Ullevaal Hospital. In 39% of the patients a major cardiac arrhythmia was recorded in the units during a mean observation time of 24 h. The subsequent mortality in the hospital was 31% in patients with arrhythmias in the MICU, and 8% in patients without arrhythmias. The association between arrhythmias and mortality was significant (P less than 0.004) in patients with a diagnosis of pneumonia. There was also an association (P less than 0.04) between arrhythmias and the severity of lung disease. Continuous electrocardiographic monitoring of patients with severe pneumonia or acute exacerbations of obstructive lung disease is recommended.