Many cancer screening studies are conducted in primary care settings, yet few systematically analyze recruitment challenges found at these sites. During a randomized trial promoting colorectal cancer screening, we implemented a process evaluation of recruitment. Recruiters maintained logs that registered the numbers of patients entering the clinic, approached by recruiters, declining to participate, and reasons for nonapproach and nonparticipation. One half of age-eligible patients were approached (n = 1,489), and half of those who met basic eligibility requirements agreed to engage further (n = 527). A small proportion of patients (n = 98) completed the 15-min assessment before their appointment. Major reasons for nonapproach included previous approach, patients called to the exam room, and appearing ill. The major reason for nonparticipation was "not interested"; a few patients did not want to share contact information. Some participants exited the assessment midway because of further ineligibility or time limitations. Best-practice recommendations for recruitment in primary care are discussed.