The purposes of this study were to determine the mental health and defenses mechanism level, to compare mental health with defenses mechanism, personal and cases data, and to compare defenses mechanism with personal and cases data among country-wide Thai inmates and prisoners.

The samples, selected from jails and prisons around country by stratified systematic random sampling, comprised of 5,274 inmates and prisoners during March to May, 2003. Data were collected by three sets of questionnaires, which were personal and case data, Thai Mental Health Indicators (66-items version), and Defense Style Questionnaire 40 (DSQ-40) respectively. Data were analyzed by using descriptive statistics, Pearson chi-square test, Mann-Whitney U test and Kruskal-Wallis test.

Most of samples, classified by types of jails and prisons, had poor mental health. Their mature defenses mechanism score was higher than neurotic and immature defenses mechanism respectively. The respondents who stay in different types of jails and prisons; who had different age, educational levels, sentenced status, numbers of crime-doing and numbers of criminal cases, had significantly different mental health level. They used different defenses mechanism significantly among different mental health, age, sex and educational level. The samples who had different numbers of crime-doing and numbers of criminal cases, and who convicted to different crimes, used some different immature defenses mechanism significantly.

The results of this study can be used as the basic benchmark for mental health development monitoring in the jails and prisons.