With the development of dental laser delivery systems that can enter into the root canals, it is possible to use Er:YAG lasers to remove the residual biofilm associated with infected root canals. We examined their effects against biofilms made of Actinomyces naeslundii, Enterococcus faecalis, Lactobacillus casei, Propionibacterium acnes, Fusobacteriurn nucleatum, Porphyromonas gingivalis, or Prevotella nigrescens in vitro. After Er: YAG laser irradiation with energy densities ranging between 0.38-0.98 J/cm(2), the biofilm samples on hydroxyapatite disks were quantitatively and morphologically evaluated. The Er:YAG laser was effective against biofilms of 6 of the bacterial species examined, with the exception of those formed by L. casei. After irradiation, the numbers of viable cells in the biofilms were significantly decreased, whereas atrophic changes in bacterial cells and reductions in biofilm cell density were seen morphologically. Er: YAG lasers might be suitable for clinical application as a suppressive and removal device of biofilms in endodontic treatments.