IntroductionThe International Classification of Headache Disorders classifies tension-type headache as infrequent episodic, frequent episodic and chronic tension-type headache [1]. The last year prevalence of infrequent episodic tensiontype headache is 64%; 61% among women and 67% among men [2]. The high prevalence causes a positive family history of infrequent tension-type headache simply J Headache Pain (2006) 7:119-126 DOI 10.1007 Are infrequent episodic, frequent episodic and chronic tension-type headache inherited? A population-based study of 11 199 twin pairs Abstract The objective was to investigate the importance of genetic and environmental factors for infrequent episodic, frequent episodic and chronic tension-type headache. Twin pairs recruited from the population-based Danish Twin Registry received a posted questionnaire. Only twin pairs where both twins replied were included. A total of 3523 monozygotic (MZ), 4150 dizygotic (DZ) same-gender and 3526 DZ opposite-gender twin pairs were included. The prevalence of frequent episodic and chronic tension-type headache was significantly more frequent in women than men, and significantly higher in those with co-occurrence of migraine. The concordance rates were significantly higher in MZ than same-gender DZ twin pairs with no or frequent episodic tension-type headache, while the difference was not significant in chronic tension-type headache. The concordance rates of infrequent episodic tension-type headache in MZ and same-gender DZ twin pairs was significantly different in women but not in men, although the difference was small in both genders. We conclude that genetic factors play a role in no and frequent episodic tension-type headache, while infrequent episodic tensiontype headache is caused primarily by environmental factors. The data regarding chronic tension-type headache were limited, so no firm conclusion could be drawn.