The authors studied the distribution of 23 histocompatibility antigens of the loci A + B in 114 patients: 26 cases of serum negative rheumatoid arthritis, and 66 variants of serum negative arthritis and 22 cases of idiopathic lumbar pain. The distribution of antigens was compared with 71 individuals in good health. The groups of serum negative rheumatoid arthritis and idiopathic lumbar pain did not show significant differences compared with a control population. In the group of "variants" the frequency of B27 was increased (p < 0.001, p corrected < 0.02) especially in those who presented spinal involvement. In psoriatic spondylitis, there was no association between B27 and the spinal involvement (p < 0.01, corrected p < 0.23).