This paper aims to identify how many young adults on antihypertensive treatment have been misclassified as hypertensive. We identified subjects aged under 35 on antihypertensive treatment, from the Health Surveys for England, 1998-2004. Pretreatment systolic and diastolic blood pressures were calculated by adjusting on-treatment blood pressures for the effects of treatment. Treatment effects were derived from meta-analysis. Subjects were classified as hypertensive if pretreatment blood pressure was >or=160/100 mm Hg, or was >or=140/90 mm Hg in conjunction with high cardiovascular risk. We then identified the proportion of treated subjects on antihypertensive treatment who were truly eligible for treatment. From the survey data we identified 65 adults (25 men and 40 women) under 35 on diuretics, beta-blockers, angiotensin converting enzyme inhibitors, calcium blockers or other antihypertensives. Average pretreatment blood pressure was 164/100 mm Hg in those eligible for treatment, and 136/79 mm Hg in those not eligible. The analysis indicated that 29.2% of adults aged 16-34 (95% confidence interval (CI): 18.6-41.8%) were truly eligible for antihypertensive treatment: 32.0% (95% CI: 14.9-53.5%) of men and 25.0% (95% CI: 12.7-41.2%) of women. A total of 73.7% (14 of 19) of subjects eligible and 41.3% (19 of 46) of subjects not eligible for treatment either had a body mass index >30 kg m(-2) or kidney disease (chi(2)-test P=0.018). Because of biological variation in blood pressure, most young adults on treatment for hypertension have been misclassified as hypertensive. Most who have been correctly diagnosed are either clinically obese or have kidney disease.