BACKGROUND: Collagen fibril contraction has been shown to be associated with tissue tightening by nonablative skin rejuvenation. Transmission electron microscopy has proven to be an effective method for characterizing collagen contraction delivered by ablative and nonablative devices used on human skin. OBJECTIVE: The purpose of this two-part study was to evaluate ultrastructural changes in cadaveric forehead skin and live abdominal skin by transmission electron microscopy for different fluence levels using the Titan infrared handpiece (Cutera, Inc., Brisbane, CA). This device is a noncoherent selectively filtered infrared device operating in the 1,100- to 1,800-nm bandwidth, intended to provide dermal heating. METHODS AND MATERIALS: Cadaveric forehead skin at 37°C was treated with a 1 × 1.5-cm spot at fluences of 50 and 100 J/cm 2. Informed consent was obtained and abdominal skin of one patient (before abdominoplasty) was treated in vivo with a 1 × 1.5-cm spot at fluences of 30, 45, and 65 J/cm 2. Punch biopsies of the treatment areas and a control area were obtained immediately after treatment. Transmission electron microscopy at depths of 0 to 1 and 1 to 2 mm was performed for each biopsy to evaluate morphologic alterations of collagen fibrils in treated areas compared to the control area. RESULTS: In the cadaveric forehead skin samples, the collagen fibril alteration was greatest in the depth range of 1 to 2 mm for both fluence settings. In the abdominal skin samples, collagen fibril alteration was not seen in the control site but was observed at all treatment levels at both the 0 to 1 and the 1 to 2-mm depths, with the least alteration seen at the shallow depth and the lowest fluence. CONCLUSIONS: Our findings suggest that collagen fibril denaturation, consistent with fibril thermocontraction, occurs immediately after infrared tissue tightening. Collagen denaturation occurs at a depth range appropriate for deep dermal treatments. The peak in collagen fibril alteration at 1 to 2 mm is consistent with contact cooling protecting the more superficial layers of the skin.