To assess the clinical utility of serum digoxin tests in the elderly and to ascertain the use by physicians of test results, digoxin levels were obtained from 77 consecutive, elderly patients (means age 72 years) taking digoxin on admission to a rehabilitation hospital. Blood samples were drawn the morning following admission (prior to the administration of digoxin) and were repeated in two weeks. Signs or symptoms of digoxin toxicity were seen in all 13 patients with levels above the therapeutic range. Of 38 patients with therapeutic levels, 16 had signs or symptoms compatible with underdigitalization or digoxin toxicity. Physicians were significantly more likely to respond to both the initial and follow-up serum digoxin test result when the first test result was above the therapeutic range.