A patient was identified with an unusual autoimmune syndrome consisting of systemic lupus erythematosus and sarcoidosis. Her serum contained extremely high levels of autoantibodies to the DEK protooncogene product. The patient's serum was used to clone a dek complementary DNA, which was expressed as a histidine‐tagged fusion protein in Escherichia coli. Using affinity‐purified recombinant DEK protein, anti‐DEK autoantibodies were found in the patient's serum at a titer of 1:10⁶ by enzyme‐linked immunosorbent assay (ELISA). Longitudinal studies revealed marked variations in anti‐DEK autoantibody levels over time. Although it has been suggested that anti‐DEK auto‐antibodies are a marker for pauciarticular juvenile rheumatoid arthritis with iridocyclitis, the present data suggest that they may be associated with other disease subsets as well. The quantitative ELISA technique will be useful for defining these subsets further and for examining the relationship between anti‐DEK titers and disease activity.