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 non-approach and non-participation. One-half of age-eligible patients were approached (n=1489), 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-minute assessment before their appointment. Major reasons for non-approach included previous approach, patients called to the exam room, and appearing ill. The major reason for non-participation was “not interested”; a few patients did not want to share contact information. Some participants exited the assessment mid-way due to further ineligibility or time limitations. Best practice recommendations for recruitment in primary care are discussed.