The actual lung cancer (screening) knowledge, attitudes, risk perceptions, reasons to participate in or decline participation, and informed decisions of subjects who decided to or decided not to participate in the Dutch-Belgian randomised controlled trial for lung cancer screening in high-risk subjects (the NELSON trial) were evaluated. A total of 2,500 high-risk subjects were asked to complete a questionnaire 3 weeks after they had received a brochure with information about the trial. Differences in knowledge, attitude and risk perception between participants and nonparticipants were analysed with logistic regression analyses adjusted for sex and smoking status. The questionnaire response of trial participants was 80% (n = 889) whereas the response of nonparticipants was low (7%, n = 97) and selective. Participants' responses to knowledge items on lung cancer as a disease were on average more often correct (mean+/-sd 68+/-17%) than items on lung cancer screening (49+/-29%). Participants had adequate knowledge on lung cancer screening (51%) more often than the nonparticipants (38%; p = 0.009). Of the decisions regarding participation, 49% were uninformed, mainly due to insufficient knowledge. Most of the participants (99%) and 64% of the nonparticipants had a positive attitude towards lung cancer screening. Additional efforts are required to improve the knowledge and understanding of subjects who are in the process of decision-making regarding participation in a lung cancer screening trial.