In 48 patients a significantly elevated plasma digoxin concentration (2.5 micrograms/l or more) correlated with the clinical symptoms and the electrocardiogram. The rhythm and conduction disturbances as well as the corrected Q-T interval and the P-T-Q index in the electrocardiogram were analyzed. In 23 patients rhythm and conduction disturbances were found. The clinical symptoms of digoxin overdose were found in 23 patients. There was no significant relationship between the corrected Q-T interval in the electrocardiogram and digoxin concentrations (r = 0.19); P-T-Q index and plasma digoxin concentrations were found to have similar values (r = 0.27), as did prolonged P-Q interval and plasma digoxin concentrations (r = 0.32). A total of 17 of 48 patients had had chronic renal failure. An S-T segment depression mostly of decreasing type was found in the whole group, suggesting that this sign was more reliable as an indicator of digitalis effect than corrected Q-T interval, P-T-Q index, or prolonged P-R interval.