Few studies have directly targeted nonparticipants in colorectal cancer screening to identify effective engagement strategies. We undertook a randomised controlled trial that targeted nonparticipants in a previous trial of average-risk subjects which compared participation rates for mailed invitations offering a fecal test, a blood test or a choice of either. Non-participants (n=899) were randomised to be offered a kit containing a fecal immunochemical test (FIT), directions on how to arrange a blood DNA test, or the option of doing either. Screening participation was assessed 12 weeks after the offer. To assess the cognitive and attitudinal variables related to participation and invitee choice, invitees were surveyed after 12wk and associations investigated using multinomial logistic regression. Participation rates were similar between groups (p=0.88): 12.0% for FIT (35/292), 13.3% for the blood test (39/293), and 13.4% for choice (39/290). Within the choice group, participation was significantly higher with FIT (9.7%, 28/290) compared to the blood test (3.8%, 11/290, p=0.005). The only variable significantly associated with participation was socioeconomic status when offered FIT, and age when offered choice but there was none when offered the blood test. Survey respondents indicated that convenience, time-saving, comfort and familiarity were major influences on participation. There was no clear advantage between a fecal test, blood test or choice of test although, when given a choice, the fecal test was preferred. Differences in variables associated with participation according to invitation strategy warrant consideration when deciding upon an invitation strategy for screening nonparticipants.