To investigate whether measuring levels of digoxin in patients with idiopathic dilated cardiomyopathy (IDC) is helpful in dose adjustment of digoxin we did the following study. In 77 patients (63 male, 14 female) with invasively verified IDC serum-digoxin levels were measured after a treatment period with beta-acetyldigoxin of at least 6 months. All patients received digoxin, 76 had ACE-inhibitors and 69 used diuretics. Mean serum-levels of digoxin were 0.96 ng/ml while using a mean daily dose of digoxin of 0.24 mg. Those who showed a serum level of digoxin within the recommended range took a slightly higher daily dose of digoxin when compared to those with low levels of digoxin (0.26 vs. 0.23 mg/day, p < 0.05). Therefore, we conclude that low serum-levels of digoxin are mainly caused by low intake of digoxin and it is justified to measure digoxin-levels in IDC-patients even if it increases costs.