We have investigated the diagnostic reliability of the immunofluorescence (IF) technique and histopathology in discoid lupus erythematosus (DLE) and lichen planus (LP) and in diseases clinically resembling these (DLE-like and LP-like). In all cases of DLE and LP it was possible to establish the clinical diagnosis with one or both methods, when in initially negative cases the investigations were repeated on fresh biopsies. In DLE the diagnostic specificity of IF was greater than that of histopathology, and the diagnostic sensitivity of the results of both methods together was greater than that of the two methods separately. In LP the diagnostic specificity of both methods was maximal, but IF showed greater diagnostic sensitivity. These differences were not statistically significant. The most important immunohistochemical feature for diagnosis by IF was the incidence and the morphological pattern of IgG along the epidermal basement membrane. This held true for differentiation between LP and DLE and also between DLE and DLE-like diseases. Combination of the results of IF and histopathology gave the most reliable results in DLE. In LP, IF was more reliable than histopathology.