OBJECTIVE:To evaluate the prevalence of isolated uncontrolled systolic blood pressure (on-treatment isolated systolic hypertension) in treated hypertensive patients and identify the characteristics and treatment strategy in these patients. METHODS:Prospective cross-sectional survey in primary care. Participating physicians enrolled more than 13 consecutive treated hypertensive patients. Patients were considered to have isolated systolic hypertension when systolic blood pressure was at least 140 mmHg and diastolic blood pressure was less than 90 mmHg. RESULTS:On-treatment isolated systolic hypertension occurred in 28% of evaluable patients (n = 11562) and in 36% of uncontrolled patients (n = 9080). Among the isolated systolic hypertension and among other uncontrolled patients, 53% and 47%, respectively, used more than one antihypertensive drug class. beta-Blockers were the most frequently prescribed antihypertensive drugs. Patients with isolated uncontrolled systolic blood pressure were more frequently treated with diuretics (43 vs. 39%) and angiotensin II receptor antagonists (23 vs. 17%). Despite blood pressure being under control in only 21% of the patients, hypertension treatment was not changed in 46% of patients with isolated uncontrolled systolic blood presssure vs. 14% of patients with both uncontrolled systolic and diastolic blood pressure. CONCLUSION:In Belgium, the prevalence of on-treatment isolated systolic hypertension in treated hypertensive patients, was 28%. The goal blood pressure was likely not reached in most patients due to inadequate treatment. The overall control rate was worse for systolic than for diastolic blood pressure. Furthermore, antihypertensive treatment was less frequently adapted in patients with isolated uncontrolled systolic blood pressure than in those patients with both uncontrolled systolic and diastolic blood pressure.