Immunofluorescence microscopy of the skin has disclosed antibodies bound to epidermal cell nuclei in several connective tissue disorders. To establish the diagnostic potential of this phenomenon the results of immunofluorescence microscopy of biopsy specimens from 1651 subjects with various diseases and from 315 patients with systemic connective tissue disorders and related diseases were reviewed. It was found that the predictive value of the phenomenon for the presence of a systemic connective tissue disorder was, in general, 88%. Except for the homogeneous and thready patterns, which seldom appear, but are specific for SLE, in vivo antinuclear antibody (ANA) does not discriminate better between the various disorders than do serum antibodies. The presence of in vivo ANA in the skin was related to serum antibodies against non-histone nucleoproteins, but not to anti-dsDNA antibodies. Combined with the finding that antibodies against non-histone nucleoproteins can bind on the surface of human keratinocytes, this suggests that ANA of the skin occurs in vivo.