The purpose of this study was to assess the physical response of skin to laser resurfacing in a real-time, quantitative fashion. The study was designed to assess skin contraction from two opposite standpoints. First, change in tension was measured during laser application while samples were held at constant length. Second, change in length of a sample under no tension was measured during laser treatment. These two disparate analyses represent the two possible extremes of the clinical situation in which skin exists under some tension with some laxity to allow for decrease in length. A custom apparatus with digital interface for skin tension measurements was used to produce single sample tracings of change in skin tension with laser treatment. Length change was measured for individual samples by continuous sonomicrometer readings. Individual sample data were then plotted in a time versus tension/length graph. Skin contracts immediately to a peak level and then relaxes to a sustained plateau level for both CO2 and erbium:YAG lasers. Increased contraction was noted when the beam penetrated into the dermis. Greater peak and plateau contraction is observed after the beam has penetrated into the dermis. Skin contraction varies directly with energy for CO2 and erbium:YAG laser. Findings were similar when skin tension was measured with the sample held at constant length and when length change was measured with the sample under no tension. Char left on the skin after a pass with CO2 laser substantially decreases skin contraction. High-density settings with CO2 laser yield pulse stacking, which effectively irradiates the same portion of tissue with char on it. Skin contraction varies inversely with computer pattern density settings for CO2 laser due to this pulse stacking effect. Density has little effect on skin contraction for the erbium:YAG laser because little char is generated. Histologic analysis identified a zone of coagulated dermis that correlates linearly with skin contraction.