The present work investigated the light and electron microscopic changes in hypertrophied gingiva in a patient with mannosidosis. The biopsy specimens studied covered a period of 20 months; biopsy specimens were taken before and after a therapeutic trial with oral and local zinc sulfate. The intensity of the disease was progressive, in spite of the zinc, and was characterized by marked hyperplasia of the epithelium and severe inflammation of the stroma. Many of the cells in the inflammatory infiltrate, as well as cells indigenous to the gingiva, showed a striking vacuolation of their cytoplasm. Histiocytes were most numerous and also were most heavily vacuolated, but fibroblasts, endothelial cells, plasma cells, and epithelial cells also manifested the vacuolar change. In the histiocytes, the vacuoles occupied most of the cytoplasm, ranged widely in size, and were contiguous, molded, and intercommunicating. The vacuoles were bound by a single membrane and were filled predominantly by a finely granular material of medium density but also by varying amounts of coarser, darker granules, fragmented membranes, myelin-like figures, lipid droplets, and small vesicles. The vacuoles were interpreted as being consistent with secondary lysosomes that contained excessively stored substrate, similar to what has been observed in the mucopolysaccharidoses, in which the vacuoles have also been demonstrated histochemically and cytochemically to contain acid phosphatase, a known lysosomal marker.