METHODS: We examined data from the Women’s Health Initiative placebo-controlled, double-blind, randomized trials. Community-dwelling women ages 50-79 years were enrolled at 40 US clinics. Women with prior arthroplasty were excluded, yielding a sample size of 26,321 subjects. Women who had had hysterectomies (n = 10,272) were randomly assigned to receive mg/day conjugated equine estrogens (n = 5,076), or placebo (n = 5,196), with a mean followup of years. Those who had not had hysterectomies (n = 16,049) were randomly assigned to receive estrogen plus progestin (n = 8,240), given as mg/day conjugated equine estrogens plus mg/day medroxyprogesterone acetate, or placebo (n = 7,809), with a mean followup of years. Participants reported hospitalizations, and arthroplasties were identified by procedure codes. Arthroplasties due to hip fracture were censored. Cox proportional hazards regression was used to assess hazard ratios (HRs) and 95% confidence intervals (95% CIs) using intent-to-treat methods and outcome of time to first procedure.