Secondary prevention of ischemic heart disease refers to the process of preventing further morbidity and reducing mortality rates in patients with clinical manifestations of the disease. Twenty-five large randomized, clinical trials addressing mortality rates and cardiovascular morbidity in patients with established ischemic heart disease are reviewed. Broadly defined, these were trials of aspirin and antiplatelet agents, anticoagulants, beta-blockers, calcium channel blockers, angiotensin-converting enzyme inhibitors, lowering of cholesterol levels, exercise rehabilitation, and diet or vitamins. In trials using warfarin sodium, timolol maleate, propranolol hydrochloride, captopril, ramipril, and simvastatin and 2 diet studies, statistically significant improvements in total mortality rates were seen. Most other studies showed non-significant reductions in total mortality rates, with statistically significant reductions in 1 or more measures of cardiovascular morbidity. The methods necessary for the reader to calculate the number (of patients) needed to treat for other studies are also reviewed. The uses and limitations of the number needed to treat as a method to compare studies of different interventions in similar populations are discussed.