This cross-sectional study used a purposive sample of 379 high school students from fifteen urban and rural high schools in Guyana and assessed their HIV and AIDS knowledge and stigma-related attitudes, and the relationships among gender, age, religion, and race/ethnicity and HIV and AIDS knowledge. Most of the high school students displayed an overall moderate level of HIV and AIDS knowledge. The students understood the modes of HIV transmission; they recognized the symptoms of HIV and AIDS; nearly half of them believed that a blood donor was at risk of contracting HIV; and about one-fifth of the students embraced myths and misconceptions surrounding HIV and AIDS.