The aim of this study was to evaluate the 1-year-period prevalence of tension-type headache in a large population based sample. The study population included 33,764 twins aged 12-41 years old from the population based new Danish Twin Registry. They received a posted headache questionnaire and the response rate was 83.5%. The self-reported 1-year-period prevalence of tension-type headache was 86.0%; 78.9% among men and 92.5% among women. The 1-year-period prevalence of infrequent episodic, frequent episodic and chronic tension-type headache was 63.5, 21.6 and 0.9%, respectively. Frequent episodic and chronic tension-type headache was significantly more frequent in women than men. The prevalence of frequent episodic tension-type headache increased slightly in men until age 39 then it declined, while it increased about 20% point in women from age 12 years to age 20-39 years old and then it declined. Congruently, the prevalence of chronic tension-type headache increased until age 39 and declined thereafter in both sexes. Chronic tension-type headache is rare in persons 12-14 years old. These effects were confirmed by age trends of the different subtypes of tension-type headache using a regression model. The prevalence of migraine varied from 7.0 to 16.8% in men and from 8.2 to 31.0% in women. It increased from age 12 to 34 years and then declined. The risk and frequency of tension-type headache was significantly higher in those with migraine than those who had never had migraine. Future large longitudinal follow-up studies are required.