OBJECTIVE - To compare the prevalence of arterial hypertension in patients with insulin-dependent diabetes mellitus (IDDM) and non-insulin-dependent diabetes mellitus (NIDDM) according to blood pressure (BP) criteria from the World Health Organization (WHO) and The Fifth Report of the Joint National Committee on Detection, Evaluation and Treatment of High Blood Pressure (JNC- V). RESEARCH DESIGN AND METHODS - Two cohorts of Caucasian patients attending the outpatient clinic at Hvidore Hospital were identified: 1) 957 patients with IDDM: 53% men, 40 (18-75) years of age (mean [range]); and 2) 549 patients with NIDDM: 53% men, 60 (20-76) years of age. BP was measured at least yearly, while patients rested in a sitting position, with a standard clinical sphygmomanometer. Patients were classified as hyper- or normotensive based on average BP during a 3-year-follow-up period. Patients receiving antihypertensive treatment were classified as hypertensives. RESULTS - In IDDM patients, the prevalence of arterial hypertension rose from 15 to 42% in the normoalbuminuric group, from 26 to 52% in the microalbuminuric group, and from 61 to 79% in the macroalbuminuric group when WHO and JNC-V criteria were compared. The corresponding rises in NIDDM patients were from 51 to 71% (normo-), from 73 to 90% (micro-), and from 82 to 93% (macroalbuminuria). Of the untreated hypertensive patients, 78% of IDDM patients and 50% of NIDDM patients had stage 1 (140-159/90-99 mmHg) hypertension; 20% of IDDM patients and 37% of NIDDM patients had stage 2 (160-179/100-109 mmHg) hypertension. Two out of three untreated hypertensive patients had isolated systolic hypertension. CONCLUSIONS - The new JNC-V criteria approved by the American Diabetes Association leads to a considerable increase in the prevalence of arterial hypertension in a population of IDDM and NIDDM patients. Isolated systolic hypertension contributes importantly.