The aim of this research was to determine whether there is a correlation between loneliness, self-esteem and social relationships (friendship and popularity) in the peer group of middleaged children, as well as whether there are differences between the genders. The research was conducted on a simple of 190 pupils (98 boys and 92 girls) attending the fourth grade of elementary school in Varaždin County. The following methods were applied: the Rosenberg Self-Esteem Scale, a shortened version of the questionnaire on loneliness, an adapted version of the questionnaire on the quality of friendship, and a measure of sociometric popularity. The results of the conducted research have shown that, on average, pupils (both sexes) have a relatively high self-esteem, low loneliness, and evaluate their friendship to be of high-quality. Moreover, self-esteem is linked to the quality of friendship and the feeling of loneliness. Lonely pupils have more negative nominations and are less satisfied with their friendly relationships than the students who estimate their loneliness as very low. Gender differences have been recorded, as the female pupils estimate their friendship with their best female friend to be of greater quality compared to the male pupils. When it comes to self-esteem and loneliness, this research has shown that the gender differences are not significant. Furthermore, gender differences were not identified in popularity, that is, group relationships.