We performed human lymphocyte alloantigen determinations in patients with psoriatic arthritis (PsPA) who were clinically classified into arthritic subtypes and disease severity. The total PsPA group demonstrated elevations of HLA-BW38, CW6, DR4 when compared with controls (p less than 0.001). Severe arthritics have increased DR4 frequency when compared with less severe arthritics (p less than 0.02). Those with mild disease have increased A3, B7 frequency which was absent in patients with moderate or severe arthritis. Rheumatoid-like PsPA patients have increased DR4, B40 frequencies; those with spondylitis, B27, CW1. PsPA is associated with many genes in the major histocompatibility complex. Distinct groups of patients with PsPA as determined by type and severity are identifiable by the presence of specific human lymphocyte antigens.