A method to determine antikeratin antibodies (AKA) is described. AKA were detected by indirect immunofluorescence technique on rat esophagus as antigen in sera of patients with definite rheumatoid arthritis (RA). The frequency of AKA in rheumatoid factor (SCAT/Waaler-Rose) positive RA was 64% and in SCAT-negative RA, 28%. Of 61 control patients with non-RA rheumatic diseases, none was AKA-positive. Of healthy controls, 2.5% were AKA-positive. In serum from 88 definite RA patients, AKA were compared with precoded clinical features. A highly significant correlation to AKA was found with the presence of rheumatoid hand deformity. Some correlation to positive SCAT titre and s-Haptoglobin was observed. Our study suggests that determination of AKA will be of value in the diagnosis of RA, especially in rheumatoid factor negative cases and that the presence of AKA indicates a more aggressive form--or results of an aggressive course--of the disease.