The Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial (ALLHAT) is a practice-based, randomized, multicenter clinical trial in 42,419 high-risk hypertensive patients aged 55 years and older; 10,356 of these patients are also in a lipid-lowering trial component. The purpose of the antihypertensive component is to determine whether the occurrence of fatal coronary heart disease and/or nonfatal myocardial infarction differs between patients randomized to diuretic (chlorthalidone) and those randomized to either calcium antagonist (amlodipine), angiotensin-converting enzyme inhibitor (lisinopril), or alpha-adrenergic blocker (doxazosin) therapy. (The doxazosin arm has been discontinued.) The purpose of the lipid-lowering component is to determine whether lowering low-density lipoprotein cholesterol with a 3-hydroxymethyl-glutaryl coenzyme A reductase inhibitor (pravastatin) in moderately hypercholesterolemic patients will reduce all-cause mortality compared to a control group receiving "usual care." ALLHAT recruited patients from a variety of practice settings from February 1994 through January 1998. Sites were paid for randomizations and are paid for completed follow-up visits and documented study events. Communication and monitoring were facilitated by nine regional coordinator teams. It was recognized from the outset that patient recruitment would be a very large task because of the number of participants (> 40,000) needed, the ambitious nature of the goal for recruitment of African-Americans (> 55%), and the knowledge that many investigators had limited experience recruiting participants for clinical trials. Multiple adjustments in the initial ALLHAT overall recruitment plan facilitated achievement of sample size goals for both components of the trial. The experience obtained from this large trial should be valuable for the planning and implementation of successful recruitment in future trials.