Clinical observations have suggested that low energy lasers might stimulate wound healing. To understand the mechanism of the biostimulation, we have previously examined the effects of low energy lasers on collagen production by human skin fibroblasts and reported an increase of collagen synthesis in vitro (J. Am. Acad. Derm. 11:1142-1150, 1980. To examine the effects of low energy lasers in vivo, hairless mice were experimentally wounded, sutured and subjected to laser irradiation by He-Ne laser with a power output of 1.56 mW, and an energy fluence of 1.22 J/cm2. Experimental wounds were subjected to laser treatment every other day, for a total duration of 2 months; control wounds remained untreated. Specimens from the wounds were then examined for histology, tensile strength and total collagen content. Results demonstrated a considerable improvement of the tensile strength of the laser-irradiated wounds at 1 and 2 weeks. Furthermore, the total collagen content was significantly increased at 2 months when compared to control wounds. These results suggest a beneficial effect of He-Ne laser on wound healing in vivo.