The utility of a new microprocessor-based method for continuous monitoring of compliance in taking solid medicaments has been evaluated. Medication intake in 31 ambulant patients was assessed in a prospective observational study under the conditions of routine practice. The patients (aged 14-87 y, mean 50 y) were receiving long-term drug treatment for various chronic diseases. There was marked interindividual and intraindividual variation in compliance with different drugs. Deviations from the prescribed dosage regimens were caused by omission of doses (22.7% of prescribed doses) and intake of extra doses (5.6% of prescribed doses). Continuous monitoring revealed that in 19% of the monitoring period no medication was taken, in 13% there was partial intake, and in 8% extra doses were taken. Patient-initiated drug holidays occurred in 50% of patients. They were responsible for 76% of the medication-free time. It is concluded, that continuous compliance monitoring is practicable in ambulatory patients. It provides information about the dynamics of drug intake behaviour that cannot be obtained from medical histories or from clinical or laboratory examination. The information could be used effectively in individual patient care and in clinical drug trials.