Objectives:
The undertaken research aimed at determining the frequency of deliberate self-injurious behaviour (D-SIB) among the students of secondary schools and also the analysis of the frequency of repeated Non-Suicidal Self-Injury (NSSI) occurrences in accordance with DSM-5 criteria in reference to the age and sex in the studied population.

Methods:
The data was collected via survey method according to the questionnaire prepared in compliance with the criteria DSM-5 and Self-Harm Inventory. The study included randomly selected students: 1193 boys and 1027 girls in Bialystok aged 12 and 19 (average age ± SD:16.8 ± 1.65). Statistical analysis of the data was carried out using the application Statistica 10.0 PL, StatSoft.

Results:
These results indicate that D-SIB and NSSI affect both sexes. In the studied group 8.3 % of students engage in deliberate self-injurious behaviour. The percentage of NSSI was 4.8% (6.3% in the group of boys, 3.2 % among girls; p(Chi2)=0.01). Self-cutting was most common among 15-year-old pupils ((D-SIB:14.75%; NNSI:8.1%). The majority of respondents (82% of girls and 74% of boys) revealed that as a result of self-injury behaviour they experience relief.

Conclusions:
Conducting further research in the area of NSSI seems to be crucial due to chronicity and prevalence as well as the fact that numerous repeated self-injuries bringing relief or causing positive state of mind might indicate a mechanism similar to an addiction syndrome in adolescence.