Numerous changes involving all organ systems are developing as a result of dysfunctional eating behavior in patients with eating disorders (ED). The changes also affect the oral cavity, in which a number of changes occur, and there are also changes in the volume and composition of saliva and in electrolyte levels. The aim of this study was to determine the differences in the frequency of changes in the oral cavity, the amount of saliva and the concentration of total amylase and electrolytes in saliva and serum, and to determine differences in nutritional status between pediatric patients with eating disorders and control groups. It was examined whether there was a correlation between the occurrence of changes and the degree of nutrition. And finally, the main factors influencing the occurrence of changes in the oral cavity in pediatric patients with eating disorders were identified. The study included 101 subjects with a mean age of 14.34 ± 1.99 years, of which 50 subjects with eating disorders and 51 subjects in the control group. Patients with PUJ were significantly malnourished and had more frequent, more pronounced and numerous changes in the oral cavity than the subjects of the control group. Higher serum concentrations of inorganic phosphates and serum magnesium, albumin, ferritin and vitamin B12 were found in patients with PUJ. Other examined parameters did not differ significantly between the examined groups. The study showed that there was an association between the changes in the oral cavity, age, duration of the disorder and the degree of nutrition, and the concentration of some of the examined parameters in saliva and serum. Recognition of oral changes by the dentist and early referral for treatment would certainly help to prevent further complications caused by the disease.