There is little support in the dental literature to justify the use of lasers for periodontal root therapy. To the contrary, there are several in vitro studies suggesting potentially adverse effects when lasers are applied to root surfaces. The purpose of this study was to evaluate, in vivo, soft tissue attachment to root surfaces following CO2 laser irradiation. Using a four-quadrant design with one quadrant serving as an untreated control, the remaining quadrants in each of two dogs were treated by (1) scaling and root planing, (2) laser only, and (3) laser followed by scaling and root planing. Prior to the assigned treatments, the roots of three teeth in each quadrant (including the control) were exposed by flap reflection and ostectomy. After root therapy the flaps were repositioned and allowed to heal for 28 days. Clinical attachment levels were determined prior to surgery, at 28 days, and by histologic measurement. Results indicate that specimens treated with laser only lost attachment compared to controls and other treatment groups. Furthermore, there was no histologic evidence of soft tissue attachment to a laser-treated surface that featured a residual char layer.