6517 Background: In NCCTG CTs a subset of AEs are assessed at each patient (pt) evaluation based on the known safety profile of agents(s). The NCCTG routinely pre-fills the “known” AE list onto CT Case Report Forms (CRFs). Newly identified AEs may expand the AE assessment list for ongoing CTs. Our survey of NCCTG AE data (JCO 2005), demonstrated that 85% of AEs reported were pre-filled on CRFs, of which, 83% did not actually occur (Grade 0). Extending this work, we evaluated the influence of pre-filling AEs on CRFs, relative to the final AE rates reported. Methods: Our non-random sample contains 507,899 AEs collected from 1/99–6/06 on 74 NCCTG CTs, 13 of which had AEs added to the CRF pre-fill list during the CT (2,604 pts, 3 Ph I/II, 8 Ph II, 2 Ph III, 9 investigational agents). Results: An average of 2.8 AEs (range 1–6) were added to CRFs for ongoing CTs, primarily for Oxaliplatin induced AEs. 58% (21/36) of AEs added during a CT were not reported prior to the addition, 22% (8) were not reported afterwards. 5 CTs had significantly higher AE rates (p<0.01) after expanding the AE list, most notably for required blood chemistries (SGOT/alk phos/creatinine/bilirubin 14.2 vs 0.72%). Overall, the same AEs were 4-fold (range 0–25) as likely to be reported when pre-filled on the CRF. Regardless of pre-filling, only 6% of the newly required AEs were Gr 3+. Conclusions: Our data suggest a significant difference between the AE rates reported if included in the CT CRF assessment list. This may significantly influence the reported results of a CT, explaining differential AE rates reported across CTs of the same agent(s). A prospective study is planned to formally evaluate this observation. Supported by NIH Grant CA25224. No significant financial relationships to disclose.