The authors evaluated the risk of venous thromboembolism associated with hormone replacement therapy in a cohort of 265,431 women aged 45-79 years who did not have major risk factors for venous thromboembolism. Through review of hospital charts, 171 cases were confirmed (pulmonary embolism = 77; deep venous thrombosis = 94). Ten thousand controls were randomly sampled. The risk of venous thromboembolism among nonusers of hormone replacement therapy was 1.3 per 10,000 women per year. Current users of hormone replacement therapy had 2.3 times higher risk of venous thromboembolism (95 percent confidence interval 1.0-5.3) compared with nonusers. The increased risk was restricted to the first year of treatment.