BACKGROUND: Evidence indicates that aspirin is effective for the primary prevention of cardiovascular disease (CVD) and colorectal cancer (CRC) but also increases the risk for gastrointestinal (GI) and cerebral hemorrhages. OBJECTIVE: To assess the net balance of benefits and harms from routine aspirin use across clinically relevant age, sex, and CVD risk groups. DESIGN: Decision analysis using a microsimulation model. DATA SOURCES: 3 systematic evidence reviews. TARGET POPULATION: Men and women aged 40 to 79 years with a 10-year CVD risk of 20% or less, and no history of CVD and without elevated risk for GI or cerebral hemorrhages that would contraindicate aspirin use. TIME HORIZON: Lifetime, 20 years, and 10 years. PERSPECTIVE: Clinical. INTERVENTION: Low-dose aspirin (