Homogeneous linear IgA deposits at the dermo-epidermal junction (DEJ) shown by direct immunofluorescence are characteristic of what is termed linear IgA bullous dermatosis. However, it is not yet certain that this disease constitutes an entity distinct from other subepidermal blistering diseases, especially when IgG deposits are also present. Sixty-one cases of subepidermal blistering disease in adults were therefore investigated by immunoelectron microscopy (IEM), and the 16 patients observed to have homogeneous linear IgA deposits were compared with the 45 who had no IgA but had IgG and/or C3. In 11 of the 16 patients with IgA (four of whom also had IgG), the deposits were linear and formed a mirror image pattern on each side of the lamina densa from which they were separated by a clear space. In contrast to this monomorphic IEM pattern, clinical and other laboratory findings were very heterogeneous, making exact clinical diagnosis difficult. Of the remaining five patients in this group of sixteen, three (all with both IgA and IgG) had bullous pemphigoid, epidermolysis bullosa acquisita, and cicatricial pemphigoid, respectively, on IEM and clinical investigation. In the remaining two patients (one with both IgA and IgG, and one with IgA only) the deposits were located in the lamina lucida, making precise classification impossible. None of the 45 patients with isolated IgG and/or C3 deposition displayed the mirror image pattern. We conclude that this IEM pattern may constitute a specific diagnostic criterion of linear IgA dermatosis.