Being a part of group, being accepted, and establishing friendships are prerequisites for a good life in childhood and in youth. Loneliness, on the other hand, has far-reaching consequences and a negative connection with mental health. Loneliness has a negative impact particularly on children and youth. Sense of coherence (concept close to sense of life control) is considered to protect the individual from stressors and to help maintain and improve health, especially mental health. Childhood and adolescence are the most important stages in developing sense of coherence. For this reason, it is important to examine factors related to sense of coherence in childhood and in youth. There is very little previous research on the correlation between loneliness and peer relations in children and adolescents and sense of coherence. Moreover, no longitudinal studies have been conducted on this topic. Purpose of this study was to describe the sense of coherence in 9th grade and the correlation between loneliness and peer relations in 4th, 6th and 9th grades and sense of coherence in 9th grade. This study was a cohort study in Finnish primary schools in a Southern Finland city. The data was collected in 2004 from 4th graders (n=985), in 2006 from 6th graders (n=985) and in 2010 from 9th graders (n=600) with electronic structured questionnaire. In this descriptive, correlative longitudinal study, loneliness and peer relations were measured in 4th, 6th and 9th grades and sense of coherence was measured with a SOC-13 questionnaire in 9th grade. Aim was to explore the associations of loneliness and peer relations at different grade levels with sense of coherence. According to the results, loneliness and problems in peer relations were associated with sense of coherence in 9th grade already from primary school. Adolescent’s sense of coherence in 9th grade was best explained in 4th grade by his/her experience of peers wanting to be his/her friend, in 6th grade by the experience of coping with peers and in 9th grade by the experience of loneliness. It is important to recognize loneliness and problems of peer relations among elementary school children as they implicate weaker sense of coherence later in their youth. Preventing and reducing loneliness in children and adolescents and promoting positive peer relations should be seen as part of efforts to improve young people's mental health and life control.