The ABCD (Appropriate Blood Pressure Control in Diabetes) trial is a large, prospective, randomized clinical trial designed to compare the effects of intensive with moderate blood pressure control on the prevention and progression of diabetic nephropathy, retinopathy, cardiovascular disease, and neuropathy in non-insulin-dependent diabetes (NIDDM). The secondary objective is to determine equivalency of the effects of a calcium channel blocker (nisoldipine) and of an angiotensin-converting enzyme inhibitor (enalapril) as a first-line antihypertensive agent in the prevention and/or progression of these diabetic vascular complications. The study consists of two study populations: a hypertensive one (diastolic blood pressure of > or = 90.0 mm Hg at the time of randomization) and a normotensive one (diastolic blood pressure of 80.0-89.0 mm Hg at the time of randomization). A total of 950 men and women aged 40-74 years were randomized and are being followed for 5 years at a single center. There were 470 randomized participants in the hypertensive population and 480 randomized participants in the normotensive population. This report summarizes the demographic, biochemical, and clinical characteristics of the randomized patients at the time of entry into the trial and evaluates the balance between the treatment groups within each population.