Objective: The prevalence of overweight and obesity was estimated among the school children and adolescents of three provinces of central Italy, and the role of several possible influencing factors was analysed., Design, Subjects and Measurements: Body mass index (BMI) was measured in 44 231 subjects, age 3-17.5 y, and a household questionnaire was filled out by the parents of 12 143 subjects to collect the following data: subjects, only child or firstborn status, prematurity, birth weight, type of feeding until the fifth month, menarche status in girls; parents, age at the time of the subject's birth; BMI (mean of the two parents) at the time the subject was measured, mother's age of menarche, socioeconomic status. BMI was measured in a subgroup of 10 795 subjects 1 y later to study the yearly sex- and age-related variations from the categories of normal weight to overweight or obesity and vice versa. All females aged 11-14 y were asked if they had their menarche., Results: Striking differences in the proportions of overweight and obesity resulted from the use of two different criteria for defining cutoff points. The overall prevalence of overweight was 13.2 and 20.7% in males, and 13.7 and 18.6% in females, and the overall prevalence of obesity varied between 24.2 and 6.3% in males, and between 22.9 and 6.1% in females, respectively. Parents' BMI, birth weight, firstborn status and post-menarche status in girls showed a significant association with overweight and/or obesity in logistic regression models., Conclusions: A large prevalence of overweight and obesity was observed in school subjects from three provinces of central Italy. From the comparisons of the prevalence rate, the new internationally agreed criteria seem more appropriate for epidemiological studies in this population., Sponsor: University of Perugia, Region of Umbria, Commune of Perugia.