An association between female sex hormones and thyroid cancer has been suggested, but the only well-established risk factor is ionizing radiation. This case-control study was designed to evaluate the association between different aetiological factors and thyroid cancer and encompassed 180 cases aged 20-70 years at the time of diagnosis in 1980-89, and 360 controls. The response rate was 92%. A history of one pregnancy gave increased risk for papillary thyroid cancer, odds ratio (OR) = 3.1, 95% confidence interval (CI) = 1.0-11, whereas no increased risk was found for > or = four pregnancies. Age > or = 25 years at the time of the first pregnancy showed a non-significantly increased risk for papillary thyroid cancer, OR = 1.5, 95% CI = 0.8-3.0. Use of oral contraceptives before the age of 20 years gave an OR of 0.5, 95% CI = 0.1-1.3. A history of asthma or allergy, or the use of cortisone, significantly decreased the risk, while cardiovascular disease in men increased the risk of developing thyroid cancer. In the logistic regression, use of cortisone decreased the risk, while one pregnancy gave a significantly increased risk in papillary thyroid cancer. The results indicated that endogenous hormones may play a role in the aetiology of thyroid cancer in women.