This study compared the plaque removal efficacy of the Philips Jordan HP510 and Braun Oral-B D9 electric toothbrushes in two clinical trials. In Trial I, a single-blind randomized split-mouth study, 24 subjects received an oral prophylaxis and abstained from all oral hygiene procedures for 24 hours before each quadrant was professionally brushed for 30 seconds. Plaque was scored before and after brushing using the Turesky-modified Quigley and Hein index (Q&H) and the approximal plaque index (API). Trial II was a two-week, single-blind balanced cross-over study comprising 54 subjects. At baseline, the subjects received an oral prophylaxis and instruction in the use of the assigned brush. Two weeks later, they returned to the clinic with 24-hour plaque accumulation. Plaque (Q&H, API) was scored before and after supervised brushing (30 sec/quadrant). After a prophylaxis, the subjects received the other brush with instructions on use, and the procedure was repeated. In Trial I, plaque was removed almost equally well by both brushes, with the HP510 and the D9 reducing Q&H from 1.46 and 1.47 (baseline) to 0.53 and 0.58, respectively, on average over all teeth. For the premolars and molars in particular, use of the HP510 resulted in statistically significantly lower Q&H scores than the D9 (0.70 vs. 0.81). In Trial II, the HP510 resulted in significantly better plaque scores than the D9 on Q&H (0.55 vs. 0.67) and API (23% vs. 28%) for all teeth. The buccal surfaces contributed the largest difference in Q&H. Statistically significant differences were also found in scores of the posterior teeth for Q&H (0.75 vs. 0.86) and API (31% vs. 38%). In conclusion, both rotating/oscillating electric toothbrushes were highly effective in plaque removal. The HP510 removed more plaque than the D9, particularly from the posterior teeth. It appeared that the smaller height and the Active Tip of HP510's brush head could have resulted in better posterior and approximal access.