Background Experience in trials of implementing quality improvement (QI) programs in nursing homes (NHs) has been variable. Understanding the characteristics of NHs that demonstrate improvements during these trials is critical to improving NH care. Design Secondary analysis of a randomized controlled trial of implementation of a QI program to reduce hospital transfers. Participants Seventy-one NHs that completed the 12-month trial Intervention Implementation included distance-learning strategies, involvement of a champion, regular submission of data on hospitalizations and root cause analyses of transfers, and training, feedback and support. Measurements Primary outcomes included all-cause and potentially avoidable hospitalizations and emergency department (ED) visits per 1000 NH resident days, and the percentage of residents readmitted in 30-days. We compared multiple other variables that could influence effective program implementation in NHs in the highest versus lowest quartile of changes in the primary outcomes. Results The 18 high-performing NHs had significant reductions in hospitalization and ED visits, whereas the 18 NHs in the low-performing group had increases. The difference in changes in each outcome varied between a reduction of 0.75 and 2.30 events relative to a NH with a census of 100; the absolute difference in 30-day readmissions was 19%. None of the variables we examined reached significance after adjustment for multiple comparisons between the groups. There was no consistent pattern of differences in nonprofit status, nursing staffing, and quality ratings. Conclusion Our experience and reviews of other NH trials suggest that key factors contributing to successful implementation QI programs in NHs remain unclear. To improve NH care, implementation trials should account for intervention fidelity and factors that have not been examined in detail, such as degree and nature of leadership support, financial and regulatory incentives, quality measures, resident and family perspectives, and the availability of onsite high-quality medical care and support of the medical director.