Increasing use of HRT over the last 2 decades could have contributed to the increasing incidence of cancer in women. Our aim was to investigate the relation between use of HRT and risk of hormone-dependent cancers in a Norwegian cohort of women. The Norwegian Women and Cancer (NOWAC) study is a representative, national, population-based cohort study. This report includes 35,456 postmenopausal women aged 45-64 years who answered a postal questionnaire in 1996-1998 providing information on reproduction, lifestyle and use of HRT. The women were followed up for cancer incidence. The main analyses were restricted to 31,451 postmenopausal women with complete information. Ever use of HRT was reported by 43.5% and current use, by 35% of the women. Current users had an increased risk of breast cancer (adjusted RR=2.1, 95% CI 1.5-2.5). The risk increased with increasing duration of use (ptrend < 0.0001). Using a regimen of continuous estrogen-progestagen implied an increased risk. Adjusted RRs associated with <5 and > or =5 years' duration of use were 2.6 (95% CI 1.9-3.7) and 3.2 (95% CI 2.2-4.6), respectively. The population-attributable risk of breast cancer due to current use of HRT was 27%. We found no significant increase in risk of ovarian cancer. Neither did we find users of estrogen-progestagen preparations to have any increase in risk of endometrial cancer. Our results suggest that HRT could be considered a major determinant for the increasing incidence of breast cancer in Norway.