Pupil discomfort in sex education lessons is high, often manifesting itself in reluctance to participate constructively. Low levels of participation are problematic, jeopardizing effective learning. Teacher interview and lesson observation data, collected in 25 schools in the East of Scotland, reveal that there is much variation between classes with regard to pupils' participation. Class size and composition (e.g. in terms of sex, ability, maturity and age), pupils' interest in sexual health matters, the teacher's relationship with the class (e.g. in terms of teacher humour and discipline), the materials used, the time of the lesson, and the school catchment were all identified as important factors. Classes with the highest level of participation tended to be comprised predominantly of girls, were older, were headed by a teacher with a strong sense of humour and a tight disciplining style and who was new to the class that year, used material which the pupils appeared to find interesting and made use of methods with which they were comfortable, and were in schools with a mixed catchment.