The ST segment in a single right-sided chest lead, CR4R, has been studied in 92 consecutive patients with acute inferior transmural left ventricular myocardial infarction. A transient ST- segment rise of more than 1 mm. was recorded in 35 patients, and strongly indicated a significant extension of the infarction to the posterior free right ventricular wall according to autopsy findings. This ECG pattern was furthermore associated with right-sided heart failure, hypotension and oliguria. Left heart failure was also common. The short-term prognosis of patients with ST-segment elevation in CR4R was poor.