Plasma digoxin concentrations on admission to hospital have been compared with levels on day 8 in a group of 50 patients who were maintained on their preadmission digoxin doses for 7 days. In the absence of a change in renal function, 18 patients (36%) had higher levels on day 8 and were considered to be non-compliant: a further 7 patients (14%) had lower levels on day 8 suggesting that before admission they had been taking more than their prescribed digoxin dose. Fifty per cent were, therefore, taking their digoxin improperly. In addition, an incorrect dose of digoxin may have been prescribed in 14 patients (28%) with plasma digoxin concentrations either below 0.8 ng/ml (1.02nmol/l) or above 2.0 ng/ml (2.56 nmol/l) on day 8. Long-term compliance was assessed by comparing day 8 'steady state' digoxin levels with those obtained at outpatient follow-up. Thirty of the original group were studied at 4 weeks when 27 per cent were considered non-compliant, and 20 at 3 months when 30 per cent were non-compliant. These results have serious implications both for drug prescribing and for the treatment of disease, and suggest that a problem of communication exists between doctors and their patients.