This pilot study evaluated, by culture testing, the effectiveness of lethal photosensitization for the microbiological treatment of peri-implantitis in dogs. Experimental peri-implantitis was induced by ligature placement for 2 months. Following ligature removal, plaque control was instituted by scrubbing with 0.12 % chlorhexidine daily for 12 months. Subsequently, mucoperiosteal flaps were elevated for scaling the implant surface. Microbial samples were obtained with paper points before and after treatment of implant surfaces by means of 100 μg/ml toluidine blue O (TBO, ) and were exposed, for 80 s, to light with a wavelength of 685 nm from a 50 mW GaAlAs diode laser. The mean initial and final bacterial counts were 7.22 ± 0.20 and 6.84 ± 0.44 CFU/ml, respectively for TVC (P < 0.0001); 6.19 ± 0.45 and 3.14 ± 3.29 CFU/ml for P. intermedia/nigrescens (P = 0.001); 5.98 ± 0.38 and 1.69 ± 2.90 CFU/ml for Fusobacterium spp. (P = 0.001); and 6.07 ± 0.22 to 1.69 ± 2.94 CFU/ml for beta-hemolytic Streptococcus (P = 0.0039). It may be concluded that lethal photosensitization resulted in a reduction of the bacterial count. Complete elimination of bacteria was achieved in some samples. (J. Oral Sci. 45, 17-23, 2003)