In this study, we sought to determine the mechanism of action of the Nd:YAG laser by examining histologic changes over time. On initial histologic findings before laser treatment, active lesions were noted to have a brisk mixed inflammatory infiltrate centered around the follicle with surrounding granulation tissue and some fibrosis. No major histologic difference was noted 24 hours after treatment from baseline; therefore, this data point was removed from our study for the remaining 12 patients. At 1 week after treatment, the superficial and deep inflammatory reaction increased, with more perifollicular inflammation and granulation tissue noted, suggesting acute effects of photothermolysis. Some patients reported an increase in tenderness and drainage during this first week, which was then followed by rapid disease improvement. This was noted to be clinically similar to a surgical incision and drainage procedure. One and 2 months after treatment, there was markedly decreased perifollicular inflammation, and the dermis was replaced by scarring and fibrosis in most patients. If present, inflammation involved the deeper dermis only and mostly spared the apocrine glands. Based on these findings, we postulate that the wavelength of the laser could not penetrate deeply enough to affect lesions in the lower dermis and dermal subcutaneous junction. However, most of the active lesions biopsied showed inflammation predominantly in the mid-reticular dermis, which may explain the effectiveness of the laser. The gross effects of the Nd:YAG laser on granulation tissue, when double pulsing, was to cause coagulation, Also, histologically there was increased coagulation of the granulation tissue. Our histologic analysis also showed that as early as 1 month after treatment, there was a significant decrease in inflammation, followed by fibrosis at 2 months after treatment. This suggests that Nd:YAG laser treatment may have resulted in the inflammatory lesions progressing more quickly through their cycle of acute, then granulomatous, inflammation to fibrosis, without the formation of abscesses and sinus tracts. Moreover, after 2 months, no recurrence of the superficial inflammation was present, signifying stabilization of disease. The Nd:YAG laser may also be decreasing inflammation by targeting water as a secondary minor chromophore and generating heat in the dermis via photothermolysis, and thus disrupting the inflammatory infiltrate. This may explain why even patients with Fitzpatrick skin types II and III and lighter hairs responded well to the laser treatments. The decrease in inflammation explains the relative decrease in the postinflammatory scarring and fibrosis that occurs as a final step in the disease progression. In addition, the photothermolysis theory also explains the improvement of the already fibrosed and scarred lesions by maximizing the tissue effects of normalization of both neocollagenesis and collagenolysis leading to optimum scar remodeling. A similar mechanism was demonstrated in a previous study32 of laser treatment of surgical scars. However, because of the small sample size of this group of patients, we could not perform a statistical analysis by Fitzpatrick type with enough power. It may be interesting to characterize the treatment response in larger subsets of patients with lighter skin types in future studies.