AIDS is a major public health problem in Nigeria. HIV counseling and testing (HCT) service is an important component of prevention and control programs. Little information is available about young persons' knowledge and use of HCT services in Nigeria. This study assessed knowledge of HIV/AIDS and use of HCT services among secondary school students and apprentices in Ibadan, Nigeria. Data were collected through 16 focus group discussions (FGD) and administration of a 36-item questionnaire to 1,281 randomly selected male and female secondary school students and apprentices aged 15-25 years. The respondents consisted of 54% apprentices and 46% students. Although virtually all respondents were aware of AIDS, only 15.8% of the sample had heard about HCT. Of those who were aware of HCT, 30.1% knew where to access the service. Students had superior knowledge of HCT than apprentices. Only 7% of those aware of HCT had ever taken the test. The percentage of those who had reportedly taken the test ranged from 11.3% of female apprentices to 6.3% of male students, 5.3% of female students, and 5.2% of male apprentices. The barriers identified by FGD participants to use of HCT services are fear of stigmatization, perceived exorbitant cost of testing, lack of knowledge of where HCT service are available, and belief that HCT is meant for only sexually active persons. However, 82.6% of the entire sample expressed the willingness to take HCT if it is provided free of charge, and there is a guarantee of confidentiality of results. Young persons have limited knowledge of HCT and under-utilize this service. Establishment of new HCT centers, promotion activities using the media, and peer education are recommended to increase uptake of this service among young persons in Nigeria.