Neurofibromas contain fibroblasts and many mast cells, and recent hypotheses have linked fibrous tissue growth to activated mast cells. We describe the ultrastructure of mast cells and fibroblasts in a case of neurofibromatosis. Mast cells were numerous and showed extensive signs of activation. Mast cells were often intimately associated with fibroblasts, and mast cell granules could be seen inside fibroblasts ('transgranulation'). The fibroblasts were also activated. These results suggest that interactions between mast cells and fibroblasts may be important in the prominent collagen production that takes place in these tumors.