Serum levels of sodium, potassium, calcium, magnesium and digoxin were studied in 67 patients on maintenance dose of digoxin, 42 with digitoxicity and 25 without. The mean serum digoxin level of toxic group was significantly higher (p less than 0.001) than non-toxic group. The mean serum potassium was significantly lower in toxic group (p less than 0.05) as compared to the non-toxic group. Of the toxic patients, 23.8% had hypokalemia. Hypokalemia resulted by significantly higher (p less than 0.005) dose of diuretic used in toxic group. The mean serum digoxin level of hypokalemic toxic group was significantly lower as compared to the normokalemic toxic group (p less than 0.001) and it was interesting to note that all hypokalemic toxic patients had their serum digoxin levels below 3 ng/ml (3.84 n mol/ml) and well within therapeutic range. There was a positive correlation between serum digoxin and potassium level amongst toxic patients (p less than 0.001). Thus, in patients on maintenance dose of digoxin therapy, use of large dosage of diuretics may result in hypokalemia, causing digitalis toxicity even at low serum digoxin levels. Serum digoxin level alone may fail as an independent guide in diagnosis of digoxin toxicity in presence of hypokalemia.