A case-control study of 1,150 female and male distal forearm cases and 2,331 controls of age 45 years and older was undertaken from 1996-2001 in five Northern California Kaiser Permanente Medical Centers. Most information on possible risk factors was obtained by an interviewer-administered questionnaire, supplemented by a few tests of lower extremity neurological function. Previous fractures since 45 years of age, a rough marker of osteoporosis, were associated with an increased risk (adjusted odds ratio [OR] [95% confidence interval] = 1.48 [1.20-1.84 ] per previous fracture). Several factors thought to protect against low bone mass were associated with a reduced risk, including current use of menopausal hormone therapy (adjusted OR = 0.60 [0.49-0.74]), ever used thiazide diuretics or water pills for at least 1 year (adjusted OR = 0.79 [0.64-0.97]), high body mass index (weight in kg/height in m2) (adjusted OR = 0.96 [0.89-1.04] per 5 unit increase), and high dietary calcium intake (adjusted OR = 0.88 [0.75-1.03] per 500 mg/day). Falls in the past year and conditions associated with falling, such as epilepsy and/or use of seizure medication (adjusted OR = 2.07 [1.35-3.17]) and a history of practitioner-diagnosed depression (adjusted OR = 1.40 [1.13-1.73]), were associated with increased risks. Having difficulty performing physical functions and all lower-extremity problems measured in this study were associated with reduced risks. The results from this and other studies indicate that distal forearm fractures tend to occur in people with low bone mass who are otherwise in relatively good health and are physically active, but who are somewhat prone to falling (particularly on an outstretched hand), and whose movements are not slowed by lower extremity problems and other debilities. Thus, measures to decrease fall frequency and to slow down the pace of relatively healthy people with low bone mass should lead to a lower frequency of distal forearm fracture.