This is an exploratory study to examine the social and cultural determinants of the teaching of HIV/AIDS sex education among secondary school teachers in Eastern Nigeria. The research analyses how teachers perceive passing their knowledge of HIV/AIDS prevention measures to their students in the context of their cultural and social norms, which restrict open discussion of sex. This is a qualitative study based on in-depth interviews with 60 teachers drawn from secondary school teachers in Eastern Nigeria, supplemented with five focus group discussions, and content analysis of teachers’ lesson preparatory notes. The findings show a high level knowledge of HIV/AIDS preventive measures among teachers. However, teachers are not passing on this knowledge because of cultural and social inhibitions. In addition, teachers have not been receiving adequate training and motivation on information, education and communication for HIV/AIDS sex education. The situation calls for serious policy intervention.