The associations between exogenous hormones, reproductive history, and colon cancer were investigated in a case-control study among women aged 30-62 years. The study was conducted in the Seattle, Washington (USA) metropolitan area between 1985 and 1989 and included 193 incident cases of colon cancer and 194 controls. There was little overall association between colon cancer and oral contraceptive use, parity, age at first birth, hysterectomy or oophorectomy status, or age at menopause. Use of noncontraceptive hormones at or after age 40, most likely hormone replacement therapy (HRT), was associated with decreased risk of colon cancer (adjusted odds ratio [OR] = 0.60, 95 percent confidence interval [CI] = 0.35-1.01), particularly among women with more than five years of use (OR = 0.47, 95 percent CI = 0.24-0.91). While results from previous studies have not been consistent, any protective effect of HRT against colon cancer would be important given the continuing debate over its potential risks and benefits.