From 1987 to 1991, over 36,000 men and women aged 20-59 years have been examined in the Monitoring Project on Cardiovascular Disease Risk Factors in The Netherlands. Classification of the treatment status of hypertensives in this population-based study was based on self-administered questionnaires. In order to assess the accuracy of self-reported antihypertensive drug use we compared the questionnaire information with computerized pharmacy records from a sample of 372 hypertensive subjects. Most antihypertensive drugs that were mentioned in the questionnaire were present in the pharmacy medication history (93%). However, this percentage was less (76%) when a comparison was made with the calculated duration of use based on the number of units prescribed and the directions for use in the pharmacy records. About 94% of the hypertensive subjects who were using an antihypertensive drug according to the pharmacy records, also mentioned at least one antihypertensive drug in the questionnaire. Agreement between self-reported antihypertensive drug use and pharmacy records was consistently high for all classes of antihypertensive drugs. Among 321 (86%) subjects, the number and types of self-reported antihypertensive drugs were exactly the same as in the pharmacy records. In conclusion, the agreement between self-reported antihypertensive drug use and pharmacy records was high, and the self-reported questionnaire information on antihypertensive drug use can be reliably used for the classification of treatment status of hypertensive subjects in this population-based study.