OBJECTIVE: To estimate the number of randomized controlled trials (RCTs) published annually that evaluate the impact of diagnostic tests on patient outcomes to gauge the extent of available randomized evidence assessing the effectiveness of diagnostic tests. STUDY DESIGN AND SETTING: Relevant RCTs published in 2004-2007 were identified from electronic searches of the Cochrane Central Register of Controlled Trials (CENTRAL). Two search strategies were developed, one using diagnostic methodological terms and one using test names. Potentially relevant RCTs were identified by screening titles and abstracts. Final inclusion decisions were based on full-text review. A random 10% sample of all citations was independently screened by a second reviewer. Capture-recapture methodology was used to estimate the number of relevant RCTs missed by both searches. RESULTS: One hundred thirty-five relevant RCTs were identified from the 23,888 records retrieved. Interobserver agreement was substantial. Capture-recapture methodology estimated that 148 (95% confidence interval: 140, 160) relevant RCTs were published in the 4-year period, an average of only 37 publications per year. CONCLUSION: RCTs of diagnostic tests that evaluate patient outcomes are rare. Consequently recommendations on the use of diagnostic tests can rarely be made on the basis of randomized comparisons, lower grade evidence frequently being the best available.