The compliance of 91 diabetic patients using oral antidiabetics was studied. Patient compliance was measured using four different methods. Patients received their medication in a Medication Event Monitoring System (MEMS)-container. Each time the patient went back to the pharmacy for refill prescriptions, the number of tablets left in the container were counted. Pharmacy records were used to study the number of days of delay in getting the next refill. At the end of the study, a questionnaire was sent to every patient. Using MEMS as a standard, the results show that pill count and refill data overestimate the compliance of this group of patients. The MEMS data also show that the compliance data using only the number of tablets may be biased, because of possible overconsumption. Pill count does not show a correlation with compliance as measured by MEMS. The relation between compliance as measured with MEMS and refill compliance is weak.