There were investigated 15 cases with normal thymus removed during cardiac surgery and nine cases with clinical signs of myasthenia gravis. Four patients with myasthenia gravis had thymoma (three invasive, one non-invasive). Specimens were fixed in buffer formalin, embedded in paraffin and slides were stained with Hematoxylin-Eosin and Alcian blue-Safranin. Additional slides were stained for factor VIII in order to estimate microvessel density. Mast cell density was performed at magnification x400, and microvessel density at magnification x200, using the "hot spot" method. There were found intralobular mast cells in all cases, located mainly in the cortex (6.53 in the normal thymus, 21.4 in patients with myasthenia gravis, and 10 in thymoma-associated myasthenia gravis). A significant increase in the number of intralobular mast cells was noticed in patients with myasthenia gravis without thymoma (p < 0.001), and a moderate increase in patients with thymoma-associated myasthenia gravis (p < 0.023). Values of microvessel density were 10.3 for the normal thymus, 33 for myasthenia gravis without thymoma and 21.8 for myasthenia gravis with associated thymoma. A strong correlation was found between the number of mast cells and microvessel densities in all three conditions.