To assess the prevalence and treatment of high blood pressure among elderly people in Mexico. A cross-sectional study was conducted from February to July 1998 among the elderly people covered by the Instituto Mexicano del Seguro Social (IMSS) healthcare services in Mexico City. The study population consisted of 4,777 subjects aged 60 years and over, selected from a cohort of 5,433 people, representative of the population of Mexico City. Trained nurses carried out three blood pressure measurements at home. Diagnosis of high blood pressure was established if systolic pressure was equal to or higher than 160 mmHg, and/or diastolic pressure was equal or higher than 90 mmHg, or by self-report of a medical diagnosis of hypertension. Demographic and risk factor information was also collected. A total of 4,777 subjects were screened; 2,036 (43%) of them reported that they had been previously diagnosed as hypertensive. Of these, 1,954 (96%) were already on pharmacological treatment. A further 273 (5.7%) subjects were found to be hypertensive at screening. Among those receiving treatment, 1,399 (68.5%) had a blood pressure reading of less than 160/90 mmHg, and this was also the case for 59 (72%) of the known hypertensives not on treatment. A single drug was used by 1,556 (79.6%) of those on treatment. Risk factors for hypertension were more frequent in the hypertensive group (p 0.05 Pound). Almost half of the elderly population is hypertensive, most of them are already on treatment, but about one third of those on treatment do not have an adequate control of high blood pressure.