The adverse effects of low-dose aspirin (100 mg daily) in the elderly were studied over a 12-month period in a double-blind, randomized, placebo-controlled trial of 400 subjects who were 70 years of age or older and had no preexisting major vascular diseases at the time of entry. Subjects were randomized so that 200 subjects received low-dose enteric-coated aspirin (100 mg daily) and 200 subjects received placebo. Compliance with medication, assessed by pill count, was 86%. Gastrointestinal symptoms were reported by 18% (n = 36) of participants receiving aspirin and 13% (n = 26) of those receiving placebo. Clinically evident gastrointestinal bleeding occurred in 3% (n = 6) of subjects receiving aspirin and none receiving placebo. Aspirin-treated subjects had a significant decrease in mean hemoglobin levels of 0.33 gm/dl during the 12-month study period, which was significantly greater than the decrease in the placebo-treated group (0.11 gm/dl; p < 0.05). These rates of unwanted symptoms are comparable with previous studies that used higher doses of aspirin. Until the risk-benefit trade-off from the use of low-dose aspirin in the elderly is established with an appropriate clinical trial, caution should be exercised when this compound is used for primary prevention of cardiovascular disease in this age group.