We examined diminutive colonic polyps to identify relationships between thermal electrocoagulation or resection trauma cytologic artifacts, type of thermal electrocoagulation, polyp size, and the interobserver variation among 3 pathologists. The 3 pathologists independently evaluated 119 colonic polyps 5 mm or less in maximum dimension for diagnosis and degree of thermal electrocoagulation or resection trauma cytologic artifacts. The maximum dimension of the polyps and type of thermal electrocoagulation were recorded. The average percentage of polyps in which a definitive diagnosis could not be made because of cytologic artifacts was 16.5% (range, 11.8%-19.3%). Decreasing polyp size was associated linearly with the inability to make a definitive diagnosis owing to cytologic artifacts. Polyps smaller than 2 mm significantly more often could not be definitively diagnosed by at least 1 pathologist owing to cytologic artifacts, including some polyps that were excised without thermal electrocautery. Interobserver variation increased with decreasing polyp dimension. Two millimeters seems to represent a cut point, below which the likelihood that a definitive diagnosis can be made can be increased if thermal electrocoagulation is used. This small size seems to make them especially susceptible to cytologically injurious forces.