Biopsy specimens from patients with systemic lupus erythematosus (SLE), discoid lupus erythematosus (DLE), and polymorphous light eruption (PMLE) were studied for the presence of ground substance changes and for thickening of the basement membrane. The criteria used to identify and classify these diseases are given. Our findings showed substantial and equal increase of hyaluronic acid in the middle and lower corium in specimens from patients with both SLE and DLE but not in those with PMLE. Thickening of the basement membrane was found in a much greater proportion in patients with lupus erythematosus (LE) than in patients with PMLE. It is concluded that histochemical studies are of value for differentiating LE from PMLE.