The purpose was to investigate the importance of genetic and environmental factors in tension-type headache using a genetic modeling analysis. Twins age 12–41 years old from the population based Danish Twin Registry received a validated posted questionnaire about tension-type headache and migraine. Inclusion required that both twins in a pairs replied on the questionnaire and known zygosity. Twin pairs where one or both twins had co-occurrence of migraine were excluded. Migraine significantly increases the risk as well as the frequency of tension–type headache. The quantitative genetic modeling included 2,437 monozygotic (MZ), 2,720 same gender dizygotic (DZ), and 2,203 opposite gender DZ twin pairs without co-occurrence of migraine. Polychoric correlations were significantly higher in MZ than same gender DZ twin pairs analyzed separately by gender, while polychoric correlation were higher in same gender than opposite gender DZ twin pairs, although this was not significant in the comparison with male same gender DZ twin pairs. The best fitting model is based on gender specific prevalence and variance components without gender specific genetic effects. Heritability estimates of 48% in men and 44% in women were obtained. Genetic effects contribute to nearly half of variance in the liability to tension-type headache. © 2007 Wiley-Liss, Inc.