Abstract: Biopsies of 24 cases of systemic scleroderma were studied by light and electron microscopy.It was found that the number of mast cells in the papillary dermis marked increased in sclerotic stage,and the mast cells displayed degranulation.The degranulated nnast cells closely contacted with functionally active fibroblasts.From these findings,the authors suggest that the pathogenesis of scleroderma may be as follows: The product of ahnornmally activated lymphocytes stimulate the proliferation of mast cells which degranulate and secrete histamine anti ather active substanccv,and these substances lead to increased fibrobiast proliferation and over synthesis of collagen.This results in mass fibrosis and the formation of scleroderma.