The topic of this thesis is Relationship between nutrition and body mass index in children of early and preschool age, and the paper is divided into two parts, the first one relates to the theoretical part, and the second covers the research. Healthy or proper nutrition, especially in children, is much more than the vital need for energy and building elements in the body. Nutrition consists of complex social and psychological elements that are crucial for the optimal growth and development of the child and for his health in adulthood. Given that healthy habits and lifestyles are formed from an early age, this is a very important topic in the early and preschool age of children. Unhealthy eating habits, irregular diet and insufficient physical activity lead to an increasingly common public health problem - obesity. Given that we are increasingly encountering overweight and obese children, it is important to work on the prevention of obesity, and at the same time emphasize the importance of proper nutrition and physical activity in children. In the research I conducted within my thesis, the aim was to explain to what extent diet affects body mass index in children of early and preschool age. The research seeks to establish what and in what way influences the formation of eating habits in children and how these same habits are related to their body mass index. The study involved 48 children aged between 5 and 7 years. The results showed that the independent and dependent variables are insufficiently related, more precisely that an unhealthy diet is not always accompanied by a BMI that goes beyond adequate nutrition. Also, children with too high BMI did not show signs of irregular meals, insufficient physical activity, or consumption of unhealthy foods. It should be taken into account that it is possible that some questions have been interpreted differently by parents, so future research should explain each question in more detail and investigate how parents prepare certain foods and how much of certain foods their children consume.