OBJECTIVE To investigate the interplay between genetic factors influencing baseline level and changes in BMI in adulthood. METHODS A longitudinal twin study of the cohort of Finnish twins (N = 10,556 twin individuals) aged 20-46 years at baseline was conducted and followed up 15 years. Data on weight and height were obtained from mailed surveys in 1975, 1981, and 1990. RESULTS Latent growth models revealed a substantial genetic influence on BMI level at baseline in males and females (heritability (h(2)) 80% (95% confidence interval 0.79-0.80) for males and h(2) = 82% (0.81, 0.84) for females) and a moderate-to-high influence on rate of change in BMI (h(2) = 58% (0.50, 0.69) for males and h(2) = 64% (0.58, 0.69) for females). Only very weak evidence for genetic pleiotropy was observed; the genetic correlation between baseline and rate of change in BMI was very modest (-0.070 (-0.13, -0.068) for males and 0.04 (0.00, 0.08) for females. CONCLUSIONS Our population-based results provide a basis for identifying genetic variants for change in BMI, in particular weight gain. Furthermore, they demonstrate for the first time that such genetic variants for change in BMI are likely to be different from those affecting level of BMI.