Epidermal nuclear Ig deposition (in vivo ANA) was observed in 54 out of 1,717 skin biopsy specimens routinely examined by immunofluorescence. Patterns were speckled (26), homogeneous (24), or nucleolar (3). The speckled pattern was sensitive (94%) but less specific (58%) for mixed connective tissue disease. The homogeneous pattern, found in SLE and scleroderma, was neither sensitive nor specific for those diseases. The nucleolar pattern was infrequently observed in scleroderma. Patterns of in vivo ANA and serum ANA did not correspond in 24% of patients. In vivo ANA were restricted to IgG class; serum ANA in 78% of patients were also of IgM class and in 37% of IgA class. The discrepancies between in vivo and serum ANA suggest that in vivo ANA is a true in vivo phenomenon.