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Background and Objective Laser treatment of port wine stains (PWS) has become an established clinical modality over the past decade. However, in some cases full clearance of the PWS cannot be achieved. To improve the clinical results, it is necessary to match the laser treatment parameters to the PWS anatomy on an individual patient basis. Therefore, knowledge of the PWS structure is of great importance. The objective of this study is to describe a diagnostic method to assess the PWS blood vessels depth and diameter from the skin surface temperature‐time course following a diagnostic laser pulse. Study Design/Materials and Methods The Monte Carlo (MC) method was used to calculate the deposited laser energy into a port wine stain skin model following irradiation by a diagnostic laser pulse at 577 nm. The heat equation was solved numerically, using the deposited energy profile as the source term, yielding the temperature‐time course at the skin surface. Subtraction of “bloodless” skin signal from that of the skin containing blood vessels gives us the net contribution of a heated dermal blood vessel to the skin surface temperature‐time behaviour. Results The net blood vessel signal shows heat‐diffusion behaviour and was found to be sensitive to the dermal blood vessel depth and diameter. The time delay for the peak signal temperature to occur depends quadratically on the blood vessel depth. The peak temperature relates linearly to the blood vessel diameter. The degree of epidermal melanin content can also be determined from the immediate temperature rise of the signal. Conclusion The proposed method easily enables assessment of the blood vessel depth and diameter as well as the epidermal melanin content in a skin model. The method can be applied to a real PWS when using the adjacent normal skin as a reference. Lasers Surg. Med. 20:179–187, 1997. © 1997 Wiley‐Liss, Inc.

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