The Consolidated Standards for Reporting of Trials (CONSORT) statement was developed to help improve the quality of reports of randomized controlled trials (RCTs). To date, a paucity of data exists regarding whether it has achieved this goal. To determine whether use of the CONSORT statement is associated with improvement in the quality of reports of RCTs. Comparative before-and-after evaluation in which reports of RCTs published in 1994 (pre-CONSORT) were compared with RCT reports from the same journals published in 1998 (post-CONSORT). We included 211 reports from BMJ, JAMA, and The Lancet (journals that adopted CONSORT) as well as The New England Journal of Medicine (a journal that did not adopt CONSORT and was used as a comparator). Number of CONSORT items included in a report, frequency of unclear reporting of allocation concealment, and overall trial quality score based on the Jadad scale, a 5-point quality assessment instrument. Compared with 1994, the number of CONSORT checklist items in reports of RCTs increased in all 4 journals in 1998, and this increase was statistically significant for the 3 adopter journals (pre-CONSORT, 23.4; mean change, 3.7; 95% confidence interval [CI], 2.1-5.3). The frequency of unclear reporting of allocation concealment decreased for each of the 4 journals, and this change was statistically significant for adopters (pre-CONSORT, 61%; mean change, -22%; 95% CI, -38% to -6%). Similarly, 3 of the 4 journals showed an improvement in the quality score for reports of RCTs, and this increase was statistically significant for adopter journals overall (pre-CONSORT, 2.7; mean change, 0.4; 95% CI, 0.1-0.8). Use of the CONSORT statement is associated with improvements in the quality of reports of RCTs.