The Coronary Drug Project (CDP) is a nationwide collaborative clinical trial sponsored by the National Heart and Lung Institute (NHLI). The primary objective is to evaluate the efficacy of several lipid influencing drugs in the long-term therapy of coronary heart disease in men of ages 30 through 64 with ECG evidence of previous myocardial infarction. The following six treatment regimens were included for study in the CDP: Equine estrogens 2.5 mg per day and 5.0 mg per day; clofibrate 1.8 g per day; dextrothyroxine 6.0 mg per day; nicotinic acid 3.0 g per day; and lactose placebo. There are 53 clinics participating in the CDP along with a Coordinating Center, Central Laboratory, ECG Reading Center, Drug Procurement and Distribution Center, and NHLI Medical Liaison Office. A total of 8341 patients were enrolled. All patients were randomly assigned to one of the six treatment groups in a double blind fashion and are being followed for a minimum of five years. The primary response variable in the CDP is total mortality; other major response variables include cause-specific mortality and various non-fatal cardiovascular events such as recurrent myocardial infarction, acute coronary insufficiency, and others. The CDP data are reviewed periodically by a Data and Safety Monitoring Committee for evidence of adverse and beneficial treatment effects. A detailed description of the study population with respect to demographic, clinical, biochemical, electrocardiographic, and other characteristics at baseline is given. In addition, some interrelationships among these baseline characteristics are presented.