In a pilot study, phase contrast microscope and plane microscope demonstrations of plaque were compared in two controlled groups to evaluate their relative effect on long-term motivation toward good oral hygiene. Plaque and calculus scores were used as indexes. During a seven-month period, the group using the phase contrast microscope showed statistically significant imporvement in plaque scores compared with the group using the plane microscope. However, the difference in actual plaque amounts between groups was of a small magnitude and cannot be considered clinically significant. Both groups showed significant statistical improvement in plaque scores at seven months as compared with pretests socres. This maintained plaquereduction at seven months was of a magnitude that could be clinically significant.