Socioeconomic status (SES) and body composition of dietary energy under-reporting (UR) and non-under-reporting youth (non-UR) were compared. Analyses covered the data obtained from 547 persons aged 15-18 years. SES discriminants included: declared economic situation of the family, the size of the place of residence, mother’s/father’s educational background and main source of mother’s/father’s income. This was used as a basis to calculate SES index and to distinguish persons of low, average and high SES. Nutritional patterns were assessed using the 24-h recall method. UR individuals were established on the basis of energy value of diets, according to criterion described by Goldberg et al. [1991]. The body composition of respondents was assessed by anthropometric methods. Due to the low participation of the UR boys, the comparison of SES and body composition between the UR and non-UR persons was performed among 278 girls aged 15-17. One hundred and thirty-six UR individuals were identified (24.8% of total sample). UR boys accounted for 4.0% of total sample, and UR girls for 20.8% of the total sample. More UR than non-UR girls revealed an average SES index level (38.6% vs. 26.8%, respectively), and less of them were characterised by a low level of SES index (21.1% vs. 35.4%, respectively). Differences in body composition and measurements between UR and non-UR individuals were established only for girls aged 15. UR girls aged 15 had a higher body weight (on average by 6.6 kg), BMI (1.8 kg/m²), hip circumference (4.0 cm), upper arm muscle circumference (1.9 cm), upper arm muscle area (652.0 mm²), fat mass (3.9 kg) and fat free mass (2.7 kg). It was found that under-reporting of energy intake from food was related to sex and SES, and in girls aged 15 years – to body weight and body composition. Energy under-reporting was definitely associated with the female sex and in girls aged 15 – by an increased body weight resulting from the increased amount of fat and muscle tissue. Proper energy reporting by girls was associated with the lowest level of socioeconomic status, which was related to living in a rural area, a low level of father’s education and a worse economic situation of the family. The results obtained suggest that for young people, reliability of a nutritional recall can be more dependent on their individual features than on the features of the social environment of their family.