This study measured the impact of an electronic Panel Support Tool (PST) on primary care teams' performance on preventive, monitoring, and therapeutic evidence-based recommendations. The PST, tightly integrated with a comprehensive electronic health record, is a dynamic report that identifies gaps in 32 evidence-based care recommendations for individual patients, groups of patients selected by a provider, or all patients on a primary care provider's panel. It combines point-of-care recommendations, disease registry capabilities, and continuous performance feedback for providers. A serial cross-sectional study of the PST's impact on care performance was conducted, retrospectively using monthly summary data for 207 teams caring for 263,509 adult members in Kaiser Permanente's Northwest region. Baseline care performance was assessed 3 months before first PST use and at 4-month intervals over 20 months of follow-up. The main outcome measure was a monthly care performance percentage for each provider, calculated as the number of selected care recommendations that were completed for all patients divided by the number of clinical indications for care recommendations among them. Statistical analysis was performed using the t test and multiple regression. Average baseline care performance on the 13 measures was 72.9% (95% confidence interval [CI], 71.8%-74.0%). During the first 12 months of tool use, performance improved to a statistically significant degree every 4 months. After 20 months of follow-up, it increased to an average of 80.0% (95% CI, 79.3%-80.7%).