Studies have reported increased psychiatric morbidity among young detainees, with as many as three-quarters reported to have one or more psychiatric disorders. Despite this, however, there is a dearth of published work among young inmates of prisons, remand homes or borstal institutions in Nigeria. The aim of this study was to assess possible psychiatric morbidity among young inmates of a borstal institution in Nigeria and to determine the factors that may be associated with this morbidity. Fifty-three inmates of one of the two existing borstal institution in Nigeria were assessed for psychiatric morbidity using the 12-item General Health Questionnaire (GHQ-12). The GHQ-12 scores were compared with the socio-demographic factors of the respondents. Twenty-eight (52.8%) of the inmates were over 18 years old, 35.8% were in the middle position within the family, 62.3% were of Christian faith, and 64.2% had their parents still living together. Seventeen (32.1%) of the inmates were from Hausa ethnic extraction, 58.5% stayed for more than 6 months at the borstal institution, and 81.1% were brought to the institution by their parents. The mean age of the inmates was 17.3 years (range, 14-23 years) and 26 (49.1%) of them were GHQ-positive. There was no statistically significant difference between the mean age of GHQ-positive and GHQ-negative inmates (F=1.73, p=0.19), and none of the socio-demographic variables were significantly associated with psychiatric morbidity (i.e. GHQ-12 positivity). The study observed a high prevalence of undetected psychiatric morbidity among inmates at the borstal institution. Efforts should be intensified by the authority responsible for managing the Nigerian prison services (including the borstal institutions) to improve mental health services.