Background: While investigators were concerned about the impact of mandated inclusion of minorities and women on the conduct of research, there has been little subsequent assessment of investigator views on the effectiveness of the mandate in achieving gender and racial/ethnic research diversity. We describe investigators' assessment of the impact of the mandate on inclusion of women and minorities in research. Methods: 440 principal investigators (PIs) surveyed between August 2002 and December 2002. Results: Most PIs (69%) felt the mandate has been successful in increasing gender diversity, 7% felt that the mandate had failed to increase females and 24% were unsure. Fifty-five percent of PIs felt the mandate had been successful in increasing minorities; 12% felt it had failed and 32% were unsure. Those who felt that failure to include women makes it hard to draw conclusions about treatment/prevention strategies (P = 0.05), white PIs (P = 0.05), and those who ascribed importance to female inclusion (P = 0.03) were more positive about the mandate's success for women. Similarly, those who felt more strongly that diversity in study samples ensures generalizability (P = 0.03), white PIs (P = 0.01), and those who ascribed importance to minority inclusion (P < 0.01) were more positive about the mandate's success for minorities. Conclusion: While the majority of PIs were positive in their assessment of the mandate on increasing diversity in research, many were ambivalent. Greater clarity on the criteria the scientific community should use in assessing the effectiveness of the mandate would allow us to reach the mandate goals. © 2006 Elsevier Inc. All rights reserved.