The Swedish Trial in Old Patients with Hypertension (STOP-Hypertension) is an ongoing multicenter, randomized, double-blind study of antihypertensive treatment (beta-blockers or diuretics) compared to placebo in hypertensive men and women aged 70-84 years. The main trial, which is now in progress, is being carried out at approximately 120 primary health care centers in Sweden, the aim being to recruit 2000 patients, who will be studied for an average period of 3 years. The main study was preceded by a 1-year pilot study in 31 centers. Almost 5000 patients were screened in the pilot study, 55% of whom were labelled hypertensive, i.e., their recumbent blood pressure was greater than or equal to 180/105 mmHg or they were already receiving antihypertensive therapy. After 1 year, 89 patients (1.9%) had been randomized to double-blind treatment and 66 patients (1.4%) were in the run-in/washout period, whereas the remainder had not been included in the trial. The most common reason for not being included as a failure to reach the inclusion blood pressure criteria (greater than or equal to 180 mmHg systolic and/or greater than or equal to 105 mmHg diastolic) following withdrawal of previous antihypertensive medication. In the main trial, recruitment of new centers and new patients has improved gradually and by April 1989 approximately 900 patients had been randomized to double-blind treatment. In an analysis of 961 elderly patients (mean age 70.1 years), the relation between systolic and diastolic blood pressure and mortality was not significant.