Guideline statements may be supported by evidence obtained from various study designs, but randomized trials are usually considered most important for making recommendations about therapeutic and preventive interventions. This study evaluated the extent to which randomized trials are cited in guidelines published in major journals. The references of 191 guidelines of therapeutic and/or preventive interventions published in Annals of Internal Medicine, BMJ, JAMA, Lancet, NEJM and Pediatrics in 1979, 1984, 1989, 1994, and 1999, were analyzed. The percentage of guidelines not citing any randomized controlled trials (RCTs) decreased gradually from 95% in 1979 to 53% in 1999. Among 4,853 references of the guidelines, there were 393 RCTs (8.1% of total), 19 systematic reviews (0.4%), and 23 meta-analyses of RCTs (0.5%). Among 19 guidelines published in 1999 or 1994 with <2 RCTs cited, in eight cases additional pertinent RCTs were identified that had not been cited by the guideline. There is a clear increase in the use of randomized evidence by guidelines over time. However, several guidelines in major journals still cite few or no RCTs.