Introduction: Fractal geometry allows the objective description of irregular objects like the human body structures. In this case it was applied to the development of a new thorax cavity characterization methodology. Materials and methods: Exploratory descriptive study where it was applied a measurement methodology based on the fractal geometry to 14 thorax x-rays from different pathologies patients. There were calculated the fractal dimensions of the thorax cavity, the cardiac silhouette and the superposition of these parts using the Box-Counting method. Results: New objective morphometric measures of the thorax x-rays with fractal dimensions were obtained. Conclusions: The fractal geometry allows the thorax x-rays mathematic characterization in patients with different pathologies. It is possible that the development of this methodology in later investigations allows to generate useful parameters of clinical application.