The cost-effectiveness of treating mild-to-moderate hypertension (diastolic blood pressures, 90-114 mmHg) was evaluated using the latest available information on both costs and benefits. The net health care costs of lifelong treatment for hypertension, at a 5% discount rate, ranged from 1491 pounds to 2752 pounds in men and from 1568 pounds to 2850 pounds in women in New Zealand in 1988 (1.00 pounds = $NZ 2.81). These figures take into account the savings in health care costs arising from stroke prevention. The net health care benefits, measured in quality-adjusted life years (QALYs) discounted at 5%, ranged from--2 days (a net negative effect of treatment) to 64 days in men and from--18 days to 35 days in women. The cost-effectiveness of antihypertensive therapy discounted at 5% (excluding categories of patients for whom the ratio was undefined due to a net negative effect of treatment on QALYs) ranged from 11,058 pounds to 63,760 pounds per QALY gained in men and from 22,060 pounds to 194,989 pounds per QALY gained in women. Treatment was more cost-effective in men than in women, in older age groups and at higher levels of pretreatment diastolic blood pressure. The cost-effectiveness ratios were highly sensitive to the discount rate used (with the majority of ratios in women being undefined at a 10% discount rate) and the costs of the regimen used (diuretic monotherapy being the most cost-effective, followed by beta-blockers, then angiotensin-converting enzyme inhibitors), as well as to the assumptions made about the impact of medication side effects on patient quality of life. These results call for a re-examination of resource allocation to antihypertensive treatment and point to the need to make assessments of the cost-effectiveness of alternative, non-pharmacological approaches to stroke prevention.