Mast cells from the upper tarsal conjunctiva of six normal subjects, seven persons with contact lens-associated persons with contact lens-associated giant papillary conjunctivitis (CL-GPC), and six persons with vernal conjunctivitis were examined by electron microscopy. Most mast cells from normal subjects were fully granulated and showed granule forms previously reported for conjunctiva. All mast cells from patients with CL-GPC had some degree of degranulation. The number of granules in approximately 30% of mast cells from patients with CL-GPC was sufficiently reduced that the cells probably would not have been recognized as mast cells using light microscopy. Mast cells from vernal conjunctivitis patients had the most extensive degranulation. The vast majority of these cells, at least 80%, had so few granules that it is unlikely they would have been recognized as mast cells if light microscopy alone were used.