To examine the extent to which the Consolidated Standards of Reporting Trials (CONSORT) reporting guidelines improved clinical trials reporting and subject attrition, which may undermine the credibility of published randomized clinical trials (RCTs). Published RCTs reported in two major medical journals before and after the CONSORT guidelines were systematically reviewed; one used the CONSORT statement (JAMA) and one did not (NEJM). The quality of RCT reporting improved for both journals, but JAMA showed more significant and consistent improvements in all aspects of RCT reporting. Subject attrition was better accounted for after the publication of CONSORT, although the attrition rates for various reasons actually increased. Attrition due to unknown reasons, as a percentage of total attrition, declined dramatically, from 68.7% pre-CONSORT to 13.0% post-CONSORT. Attrition of study subjects remains a serious problem in RCTs. Bias from selective attrition can undermine the presumptive scientific advantage of RCTs. The CONSORT guidelines improved RCT reporting when they were implemented but did not substantially improve reported attrition rates.