We investigated recruitment into a randomised controlled trial (RCT) following contact with a generalist telephone counselling service. Telephone counsellors were required to ask callers if they would be willing to receive a telephone call to discuss their possible involvement in a trial designed to evaluate the use of a web-based mental health intervention. The five centres involved answered a total of 34,722 counselling calls during the recruitment period. Only 9% of callers were invited by the telephone counsellor to receive the recruitment call and of all callers, only 2.6% agreed. Common reasons reported by the telephone counsellors for not referring eligible applicants to the trial included their view that it was inappropriate to ask the caller, feeling uncomfortable about asking the question, being concerned that the invitation might affect the counselling relationship and forgetting to ask. The presence of an on-site trial manager, regular encouragement and feedback, and tailored and specific recruiter training may improve rates of recruitment in future trials.