OBJECTIVE To analyse the efficacy of health education (HE) through a group session with two years of intervention by postal back-up on compliance with therapy for light-to-moderate essential hypertension. DESIGN Controlled, randomised clinical trial. SETTING La Orden Health Centre. PARTICIPANTS 110 hypertense patients diagnosed with de novo or unmonitored hypertension. INTERVENTION 1. Control group (CG): 55 patients who received HE from their doctor. 2. Intervention group (IG): 55 patients who also received a controlled intervention. a) Group HE session, b) Postal back-up to the home every three months for two years. MEASUREMENTS AND RESULTS After two years, on two surprise home visits made at a month's interval, blood pressure was measured and pressure-lowering pills counted. The person whose consumption was at 80-110% of the amount prescribed was defined as compliant. The percentages of compliant patients and mean compliance were compared for the two visits. The reduction of absolute risk (RAR), of relative risk (RRR) and the number of patients needed to avoid non-compliance (NNT) were calculated. 92 people finished the trial. The two groups (n: CG = 45, IG = 47) did not differ for age, sex, time of evolution, number of illnesses or dosages of medicines prescribed. 78.26% were compliant (CI: 61.5-94.9), CG = 55.6% and IG = 95.7% (p < 0.0001). Mean overall compliance was 90.9 +/- 14, CG = 88.1 +/- 12 and IG = 93.86 +/- 11 (p = 0.01). RAR was 40.1%, RRR 90.3 and NNT 2.49% respectively. CONCLUSIONS HE intervention is an effective measure to improve therapeutic compliance in essential hypertension and lasts for up to two years.