The objective of the study was to identify factors related to the number of solution-focused brief therapy sessions required to solve adolescents' problems. The study was conducted at the foster care and health care institutions. The sample consisted of 73 adolescents (41% of males, 59% of females), aged 12 to 18 years, who achieved high level of therapeutic progress during solution-focused brief therapy. Respondents from foster care institutions made up 47% and from health care institutions--53%. The study design included: (1) an initial evaluation, where adolescents' psychosocial adjustment and personality traits were evaluated as well as information on demographic characteristics and type of referral for therapy was collected; (2) solution-focused brief therapy was carried out. In the first session, information on the type and severity of the problem presented for the therapy and motivation to solve the problem was collected; (3) the effectiveness of solution-focused brief therapy was evaluated. Standardized interview for the evaluation of psychosocial adjustment of adolescents was used to evaluate the difficulties of adolescents' psychosocial functioning. Eysenck Personality Questionnaire was administered to evaluate adolescents' personality traits. Therapist's evaluation of improvement was used to evaluate the effectiveness of solution-focused brief therapy. The analysis of results showed that 60.3% of adolescents needed two to three solution-focused brief therapy sessions to solve their problems. Lower number of sessions needed to achieve a solution was related to lower level of psychoticism, lower level of subjectively evaluated problem severity, and living with parents (as the opposite of living in foster care institutions). Ordinal regression analysis revealed that living with parents, self-referral to the therapy, lower level of subjectively evaluated problem severity, and higher self-confidence were significant predictors of lower number of sessions needed to achieve solution.