Purpose: To test a system of measuring early plaque formation (EPF) and its subgingival extension as related to the presence or absence of a plaque free zone (PFZ). Materials and Methods: EPF was measured by three independent examiners following two consecutive 72-hour periods of undisturbed plaque build-up. One of the examiners further measured EPF following a 96-hour period in the presence of chlorhexidine or placebo. The classification system was composed of criterion 0 (plaque-free dental surface), criterion 1 (presence of plaque and PFZ) and criterion 2 (absence of PFZ, subgingival extension of plaque). Intra- and inter-examiner reliability were evaluated by means of the percentage of absolute agreement (c), Kappa (k) and Kendall (kd) coefficients. The third experiment consisted of a double-blind, placebo-controlled, cross-over trial. Plaque build-up in the presence of 0.12% chlorhexidine was assessed by employing the classification system described. Results: The percentage of absolute intra- and inter-examiner agreement ranged from 85.43% to 75.63% and from 77.31% to 75.35% respectively. Chlorhexidine and placebo rinses showed similar percentages of criterion 1 surfaces, 62.6% and 51.5% respectively (p = 0.343). Of the surfaces, 44.3% showed criterion 2 after the use of placebo, while 3.4% of surfaces showed this criterion with the chlorhexidine (p = 0.007). Conclusion: The events associated with EPF can be appropriately scored with this classification system. Chlorhexidine rinses inhibit both the plaque colonization of the dental surfaces as well as its subgingival extension. Keywords: dental plaque/diagnosis, dental plaque/prevention and control, early plaque formation, reliability