Background: DISA (Depressive Symptoms In Swedish Adolescents) is a cognitive behavioral intervention aimed at preventing stress and depressive symptoms in adolescents. It is frequently used in Swedish schools for students aged 13-15 years. DISA is commonly offered to females, but at some schools, the intervention is also offered to males. In this study, the application of the intervention for both females and males is evaluated.

Aim: The overarching aim of this thesis was to explore the experience, significance, and effectiveness of the school-based intervention DISA. The specific aims were to investigate the effectiveness of depressive symptoms and self-reporting health, and costs and cost-effectiveness as well as to elucidate the experiences of adolescents and tutors.

Methods: The effectiveness of DISA was investigated using method triangulation. The thesis is based on two quantitative and two qualitative studies. The quantitative studies were quasi-experimental trials with an intervention group (I, IV), and a control group (IV), with follow-up measurements obtained at 3 and 12 months after baseline. The qualitative studies were based on focus group interviews with adolescents (II) and tutors (III). The school-based cognitive behavioral prevention program DISA was presented by school health staff and teachers once per week for ten weeks. Students in grade 8 (median age: 14) participated in the intervention in study I (n=62, 52% females) and study IV (n=462, 79% females), and 486 students (46% females) were allocated to the control group (IV). Focus group interviews were conducted with 89 adolescents (II) and 22 tutors (III). The interviews were analyzed with qualitative content analysis.

Findings: The intervention group decreased their self-reported depressive symptoms and improved their self-rated health more than the control group (p<0.05) at the 12-month follow-up. The majority of the adolescents rated the cognitive behavioral program as a positive experience, and the attendance rate was high. The incremental cost-effectiveness ratio was approximately USD 6,300 per quality-adjusted life year (QALY). The focus group interviews with the adolescents revealed that the students found that they developed intrapersonal strategies through DISA, such as directed thinking, improved self-confidence, stress management, and positive activities. They also gained an increased interpersonal awareness regarding trusting the group and considering others. However, structural constraints of the DISA program, such as negative framing and an emphasis on performance, were also noted. The focus group interviews with the tutors identified an overall theme of striking a balance between strictly following the manual and meeting student needs.

Conclusions: The DISA program appears to be a feasible, cost-effective school-based cognitive behavioral program with high levels of student adherence and satisfaction, as well as positive mental health benefits. However, a desire for a more health-promoting approach was expressed.