We assessed the determinants of onset of hypertension in a large, prospective population-based study of perimenopausal women from the Kuopio Osteoporosis Risk Factor and Prevention (OSTPRE) study. The data collection started in 1989, when a baseline postal inquiry was sent to all women aged 47-56 years (n=14 220) residing in the Kuopio Province in Eastern Finland. Names, social security numbers and addresses were obtained from the Population Register Centre of Finland. A total of 11 798 women responded at baseline and at 5-year follow-up in 1994. After the exclusion of 1777 women with prevalent hypertension at baseline and women with missing height or weight information, the study population consisted of 9485 without established hypertension at baseline. New cases of established hypertension during the follow-up (n=908) were ascertained with the Registry of Specially Refunded Drugs of the Finnish Social Insurance Institution (SII). According to the National Health Insurance, the SII granted 90% reimbursement for drug costs in defined chronic illnesses necessitating continuous medication, like arterial hypertension. Weight and weight gain both raised the risk by 5% per kg (P<0.001). Weight gain of 4-6 kg increased the risk of hypertension 1.25 times and a gain of more than 7 kg 1.65 times compared with the control (zero) group. To conclude, the onset of hypertension in peri- and early postmenopausal women was related to an increase in body weight despite controlling for initial body weight, reported physical activity and use of HRT. Therefore, preventing weight gain by dietary means and exercise is of great importance at menopausal age.