Medullary thyroid cancer is a rare tumour that appears in two distinct forms. The rarer familial form is genetically determined. The sporadic form is more common, but its aetiology has not been defined clearly so far. The aim of this study was to examine the risk factors for development of sporadic medullary thyroid cancer (sMTC). A case-control study was carried out during the period 2000-2009. The case group included 98 consecutive patients with sMTC. The control group comprised twice as many cases (196), who were neighbours of the patients from the case group. Patients were individually matched by sex, age and place of residence. Conditional univariate and multivariate logistic regression methods were applied in data analyses. According to the univariate logistic regression method, sMTC was significantly related to smoking status, duration of smoking, number of cigarettes smoked per day, personal history of goitre or thyroid nodules, personal history of nonthyroid cancer, menarche after 14 years of age, first full-term pregnancy before 20 years of age and usage of oral contraceptives. According to the multivariate logistic regression method, sMTC was independently related to smoking status [odds ratio (OR)=0.46, 95% confidence interval (CI)=0.20-0.90], personal history of goitre or thyroid nodules (OR=11.29, 95% CI=1.16-73.45) and menarche after 14 years of age (OR=2.77, 95% CI=1.33-6.28). Risk factors for sMTC were goitre or thyroid nodules and late menarche; cigarette smoking appeared to be a protective factor.