1. The National Blood Pressure Study (NBPS) is a single blind trial designed to test the efficacy of active drug treatment in reducing complications from mild hypertension (mean diastolic pressure = 95–109 mmHg).

2. Between 1973 and 1975, four centres screened about 104 000 subjects aged 30–69 years, yielding an estimated prevalence of hypertension (≥95 mmHg diastolic) of 16% and of moderate-to-severe hypertension (≥110 mmHg diastolic) of 1·3%.

3. Some 4000 subjects selected for untreated uncomplicated mild hypertension were randomized to either active treatment (chlorothiazide + α-methyldopa and/or a β-adrenoreceptor antagonist as required) or to matching placebos.

4. At 1 year mean pressures had fallen significantly below entry pressures in both groups but in the active group the fall was greater by a margin of 14·4±1·3 (sem) mmHg systolic and 71 ±0·7 mmHg diastolic. At 1 year 5% of subjects in the placebo group had been placed on active treatment on the ethical grounds that pressure had exceeded the mild hypertension limit.

5. Trial end-points (death, morbidity from stroke, hypertensive heart and renal disease, and ischaemic heart disease) number 106 (nine deaths) thus far, of which ischaemic heart disease accounts for 71% and stroke 19%.

6. The duration of trial may need to be extended beyond the original estimate of 5 years.

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