[The fractals theory and its significance for dermatology].
Classic mathematical methods are frequently not suitable for the investigation of complex "natural" shapes, that cannot be approximated by geometric structures. A theory developed by Mandelbrot has made it possible to analyse such "fractals". Many biological and medical shapes could be identified as fractal. These results suggest fractal structures for some dermatological lesions. This is particularly true for skin lesions related to the vascular system (e.g., livedo racemosa, spider naevus). Furthermore, computer simulation of pathogenetic mechanisms can demonstrate that the lesions of a skin disease are fractal in nature. This method can be applied to skin tumours with horizontal cell growth (e.g., carcinoma and melanoma in situ) and to the architecture of spider naevus.