The Kuopio Osteoporosis Risk Factor and Prevention (OSTPRE) Study examines the risk factors of osteoporosis, the relationship of risk factors to bone density and fractures, as well as the possibility to prevent bone loss by administering certain hormones. The baseline postal enquiry in 1989 was sent to all the 14,220 women aged 47-56 years residing in the Kuopio Province, Finland. The questionnaire included questions about their gynaecological history, physical exercise and smoking habits, calcium intake, body weight and height, history of bone fractures, health disorders, their current and previous use of drugs, as well as their willingness to participate in bone densitometry and in a clinical hormone trial. The response rate was 92.8%. In all, 56% reported some previous use of female hormones. Strong contraindications for oestrogen replacement therapy were found in 9.3% of the women. Almost half of the respondents reported lack of regular physical exercise, 11.9% were smokers, and 17.0% reported a calcium intake from milk products of less than 500 mg daily. The incidence of fractures increased steadily with age. The incidence of premenopausal fractures within the last 10 years was 7.65 per 1000 person/years and that of postmenopausal fractures was 17.40 per 1000 person/years (P = 0.000). The effect of menopause on fracture incidence was stronger than the effect of a 5-year age increase. Of the respondents, 84.4% were willing to participate in bone densitometry and 68.3% for long-term prevention of osteoporosis with oestrogen.