To assess the influence of oral contraceptives (OC) on the risk for venous thromboembolism (VTE) in young women, a 5-year case-control study including all women 15-44 years old suffering a first deep venous thrombosis or a first pulmonary embolism from all Danish hospitals, along with 1200 control subjects during the period 1994-1995, was conducted. Of 586 patient and 1200 control subject questionnaires sent out, 523 patient (89.2%) and 1074 control (89.5%) questionnaires were returned with an agreement to participate. After exclusion of women with nonvalid diagnoses, women who were pregnant, and women with previous VTE or acute myocardial infarction (AMI), 375 patients and 1041 control subjects were available for analysis. Potential tested confounders included: body mass index, length of OC use, family history of VTE, AMI, or stroke, smoking habits, coagulopathies, diabetes, years of schooling, certainty of diagnosis, previous births, and treated hypertension during any pregnancy. A multivariate analysis was performed. Estrogen dose had no influence on the risk for VTE. The risk for VTE among current users of OC was primarily influenced by duration of use, with significantly decreasing odds ratios (OR) over time: < 1 year; 5.1 (3.1-8.5); 1-5 years; 2.5 (1.6-4.1); and > 5 years; 2.1 (1.5-3.1), all compared with those for nonusers of OC. This trend was still significant after adjustment for progestin types. Without adjustment for duration of use, current users of OC with second generation (levonorgestrel or norgestimate) and third generation (desogestrel or gestodene) progestins had OR of 1.8 (1.1-2.9) and 3.2 (2.3-4.4), respectively. After correction for duration of use, however, no significant differences were found between users of OC with different types of progestins. In conclusion, OC increase the risk for VTE significantly. The risk among current users of OC is primarily influenced by duration of use. No difference in risk was found according to estrogen dose, and the difference in risk between different types of progestins was not statistically significant after adjustment for duration of use.