OBJECTIVE: The aim of this study was to estimate the proportion of strokes occurring among hypertensive patients in The Netherlands attributable to under-treatment of hypertension. METHODS: The baseline data reported in this study were collected in two population-based studies. Approximately 45,000 men and women aged 20 years and over were examined in these studies. In all participants, blood pressure, serum lipid levels, weight, and height were measured, and all respondents completed a questionarre on cardiovascular risk factors and current use of medication. For the classification of under-treatment, the Dutch guidelines for management of hypertension were used (pharmacological treatment when diastolic blood pressure [DBP] is between 100 and 105 mmHg and one or more cardiovascular risk factors are present, or when the systolic blood pressure [SBP] is between 160 and 180 mmHg and one more cardiovascular risk factors are present, or when DBP > 105 mmHg, or when SBP > 180 mmHg; treatment goal is a DBP < 90 mmHg and a SBP < 160 mmHg. Information on fatal and non-fatal strokes during follow-up were obtained through patient records from general practitioners. RESULTS: Of the subjects, 3168 were hypertensive. Subjects who were not in need of pharmacological treatment according to the guidelines and subjects with missing data were excluded. Follow-up was complete for 2369 (91%) of the 2616 remaining subjects. Mean duration of follow-up was 4.6 years. Compared to treated and controlled hypertension, the adjusted relative risks of stroke were 1.30 [95% CI 0.70-2.44] and 1.76 [95% CI 1.05-2.94] for treated and uncontrolled hypertension, and untreated hypertension that should be treated, respectively. Among hypertensives the proportion of strokes attributable to under-treatment of hypertension was about 28%. CONCLUSION: Improvement of control of hypertension among treated hypertensives, detection of hypertension, and adherence to the current guidelines on the management of hypertension might prevent a considerable proportion of the incident strokes among hypertensives.