Studies on the anti-keratin intermediate filament autoantibodies (anti-KIF-Abs) in sera from psoriasis (Pso) patients were performed by immunoblotting and enzyme-linked immunosorbent assay (ELISA). According to their reactivities against keratin subunits (50, 56.5, 58, and 63-68 kd), sera were divided into four groups. However, no significant differences in these reactive patterns were found between healthy volunteers and Pso patients. In the second experiment, anti-KIF-Abs in sera from Pso patients, pustulosis palmaris et plantaris (PPP) patients, atopic dermatitis (AD) patients, systemic lupus erythematosus (SLE) patients, and healthy volunteers were determined by ELISA, using as substrate keratins purified from normal human stratum corneum and 48- and 50-kd keratins purified from psoriatic stratum corneum. The serum titers of anti-KIF-Abs against 48- and 50-kd keratins in Pso patients were significantly higher than those in PPP patients, AD patients, SLE patients, or healthy volunteers. The elevated titers of anti-KIF-Abs against the 48- and 50-kd keratins in sera of Pso patients showed a significant decrease with improvement of psoriatic lesions. The above results suggest that anti-KIF-Abs against 48- and 50-kd keratins in sera of Pso patients have some relevance to the severity of the disease and can be used as a marker for the evaluation of the disease activity of psoriasis.