The morphology of human hair was extensively discussed in the last century, except for hair medulla, mainly because it was believed to have little or no influence on any useful hair property. Early SEM results showed that medulla is formed by unorganized fibrilar material that could be macrofibrils randomly located in the fiber center. The present paper aims to correlate the fibrilar structures with the macrofibrils using transmission electron microscopy (TEM) and to evaluate the influence of medulla on the mechanical properties of hair. TEM micrographs show that the interface between cortex and medulla is surrounded by a CMC layer and that there is less electronically dense material between cortical cells. Cortical cells in medulla give the usual microfibril crystalline arrangement. The cells become scarce and less organized in the center of the medulla, which also shows air filled granules. Average values of the mechanical properties are similar for unmedullated and medullated fibers. However higher dispersion in data for medullated fibers is observed. Unmedulated fibers are more uniform and show smaller diameters. These data indicate that the air cavities in medulla do not interfere with the mechanical properties, but leave hair strength less uniform.