From 1996 to 2001, the authors undertook a case-control study of 192 pelvis fracture cases (men and women) and 2,402 controls aged > or = 45 years at five Kaiser Permanente medical centers in Northern California. Most information on potential risk factors was obtained by means of an interviewer-administered questionnaire. Number of fractures since age 45 years and a maternal history of hip fracture were associated with increased risks. Several factors thought to protect against loss of bone mass, including recent use of menopausal hormone therapy (adjusted odds ratio (OR) = 0.55, 95% confidence interval (CI): 0.33, 0.91) and high body mass index (weight (kg)/height (m)2; per 5-unit increase, adjusted OR = 0.65, 95% CI: 0.52, 0.81), were associated with decreased risks, while cigarette smoking (adjusted OR = 2.17, 95% CI: 1.34, 3.52) and hysterectomy (adjusted OR = 1.75, 95% CI: 1.15, 2.66) were associated with increased risks. Various conditions related to propensity to fall were associated with increased risks. Most indicators of frailty, including use of walking aids and needing help with or being unable to perform various activities of daily living, conferred increased risks. Thus, low bone mass, frailty, and probably a propensity to fall appear to be associated with increased risk of pelvis fracture.