Objective: Childhood overweight and rapid weight gain during the first months of life have been shown to be major risk factors for the development of later overweight. Studies in children show that there are temperamental risk factors for the development of overweight, but little is known about early infancy. Methods: The present study investigated the relationship of infant difficult temperament, assessed at age 6 months, with overweight status at birth and at 6 months of age and with rapid weight gain during this period. Data collection was conducted as part of the Norwegian Mother and Child Cohort Study at the Norwegian Institute of Public Health (1999-2006). The analyses are based on data retrieved from the Medical Birth Registry of Norway, health charts, and maternal reports during pregnancy and when the child was 6 months of age. After application of eligibility criteria, 29,182 infants could be included in the study. Results: In adjusting for infants' sex, formula feeding, maternal age, body mass index and diabetic status, and parental duration of education, infant difficult temperament was slightly negatively associated with overweight status at birth but not at age 6 months. In addition, infant difficult temperament was slightly positively associated with rapid weight gain during the first 6 months of life. Conclusions: Despite statistical significance, these associations do not appear to be clinically relevant. Future studies should explore whether the impact of temperament increases with age.