The purpose of this study was to evaluate the effects of Nd:YAG laser treatment on root surfaces in vitro when used alone or in combination with conventional scaling and root planing. The study population consisted of 18 unerupted third molars, each with a 3 mm diameter treatment site outlined on the root surface. Three specimens were randomly assigned to each of 6 different treatment groups: 1) untreated control; 2) root planed only; 3) laser treated only using 1.25 W of energy; 4) laser treated only using 1.50 W of energy; 5) laser treated with 1.25 W of energy followed by root planing; and 6) laser treated with 1.50 W of energy followed by root planing. Following their respective treatment, all specimens were prepared for evaluation by scanning electron microscopy. Specimens from Group 2, root planed only, exhibited a smear layer of scale-like texture with parallel instrument tracks resulting from curet use. Specimens treated by laser only, Groups 3 and 4, featured various surface changes not observed in controls such as charring and carbonization of the cementum surface, randomly distributed pitting and crater formation, and melting of the root mineral phase with subsequent resolidification as porous globules. Those specimens treated by laser followed by root planing, Groups 5 and 6, exhibited surface characteristics similar to those noted in Group 2 specimens. Further, there were areas of exposed dentinal tubules resulting from a "peeling" of the cementum layer. The results of this in vitro study suggest that laser use during root preparation, even at relatively low energy levels, will result in physical changes to the root surface.(ABSTRACT TRUNCATED AT 250 WORDS)