The aim of this 5-year longitudinal study of 593 girls (9-20-year-old) was to examine whether the internalization of the thinness ideal in terms of 'a wish to be thinner' might be related to lifestyle factors and longitudinally increase the risk of disturbed eating over time. Results showed that a wish to be thinner was related to lifestyle factors, eating attitudes and body mass index (BMI) longitudinally. Girls who wished to be thinner dieted more often, thought that they would be more popular if they were thinner, skipped meals, were eating breakfast more often alone and had a higher BMI compared to the girls without such a wish. Girls who wished to be thinner were 4 times more likely to develop disturbed eating attitudes over a 5-year period. These findings point to the importance of helping adolescents to establish regular eating habits, to avoid unhealthy dieting practices and to prevent sedentary behaviours that might lead to overweight and or obesity in early childhood.