We estimated genetic and environmental components of variance of body mass index (BMI) among 7245 same-sexed Finnish MZ and DZ twin pairs aged 18-54 years from the nationwide Finnish Twin Cohort. Age accounted for 20 per cent of variance among men and 26 per cent in women. The contributions of additive genetic effects, shared and non-shared environmental effects on BMI-variance were estimated by LISREL structural equation models. Genetic effects accounted for 72 per cent and 68 per cent of total variance in men and non-pregnant women respectively, while 28 per cent of variance among men and 32 per cent among women was due to non-shared environmental effects. This gender difference was statistically significant. Models including shared environmental effects did not improve model fits. The magnitude of the genetic component of BMI was also analysed separately for each 10-year age group. Models with age-specific parameters for genetic and environmental effects fitted significantly better than models with effects constrained to be equal over age. Our results indicate a substantial genetic component in BMI. However, the magnitude of additive genetic effects decreases with age in both genders.