To explain the mechanism of action of a novel intralesional cryoprobe, thermal behavior measurements, and histological studies were performed in swine muscle specimens after intralesional cryosurgery ex vivo. Slow cooling (20 degrees C/min) and thawing (25 degrees C/min) rates, end temperature of -30 degrees C, produced 8 mm wide diffuse coagulative-type necrosis and a 3 mm-wide transition zone around the cryoprobe. In contrast, contact cryosurgery showed fast cooling and thawing rates (80 degrees C/min) and an end temperature of -100 degrees C. Efficacy and safety of the intralesional cryoprobe was further assessed in ten recalcitrant auricular keloids in nine Caucasian patients. There was a 67.4 +/- 23 percent reduction of scar volume at the end of the 18-month follow-up period after a single intralesional treatment (p < 0.005). Significant reduction of hardness, elevation, and redness as well as itching, pain, and tenderness was documented. The histomorphometric analysis, including spectral and fractal analysis, as well as assessment of the fast Fourier transform algorithm, showed parallel alignment and reorganization of the collagen fibers in the treated scar similar to that in the normal dermis. A long hold time in the deep scar core caused minimal damage to the superficial tissue, including melanocytes. There was no evidence of permanent hypopigmentation, active bleeding, infection, or recurrence. The major advantages of the intralesional cryoprobe, including the marked efficacy of a single treatment, may have a major importance in the clinical application of cryosurgery in the treatment of keloids and of other lesions localized deep in the skin.