To evaluate the knowledge, attitudes and sexual behaviour with respect to HIV and AIDS among Kenyan secondary school students, a questionnaire was issued to 3,018 students of mean age 16.3 years in 11 Kenyan schools. Questions of knowledge were answered correctly by an average of 77.1% of students. Areas where students' knowledge was less complete included the inability of mosquitoes to transmit the virus, the protective effect of condoms, the lack of protection from medications, the fatal and incurable nature of AIDS, and the fact that those infected with HIV may appear healthy. No prior sexual experience was reported by 71.3% of females and 25.2% of males. Multiple sexual partners were reported by 41.2% of males and 7.3% of females. Sixty per cent of students denied ever using condoms during sex and only 6.8% of those with multiple partners used them all the time. A prior sexually-transmitted disease was reported by 5.6% of students. Although a high level of knowledge regarding HIV and AIDS is evident among Kenyan students there is a sizable number who admit to extensive sexual experience, but who are not using condoms, thereby putting themselves at risk.