This article summarizes the results of an investigation, which deals with the development of overweight and obesity of children at the age before primary school. In three age groups, 10% of the children in Dortmund, Germany, were tested in a representative random sample (a total of 1,323 children). At the age of 3 years, body weight, height, and motor competencies were measured. The relevant data from the U1 ("Untersuchung 1", check-up 1) and U6 ("Untersuchung 6", check-up 6) were taken from the preventive medical check-up booklet. In addition, the parent present at the examination was measured and interviewed about their social and cultural background. The comparison of the reference values of Kromeyer-Hauschild and from the KiGGS study ("Kinder- und Jugendgesundheitssurvey") results in comparable evaluations of the weight status. For the ages birth and 1 year, there are no differences compared to the reference values. At the age of 3 years, the prevalence of overweight and obesity is significantly higher. The majority of children who were overweight or obese at the age of 1 year were also overweight or obese at the age of 3 years. Significant factors influencing overweight are the social demographics and the weight of the parents. The development of early infantile overweight can, thus, be attributed to social and cultural effects.