Forty-three psoriatic patients with spondylitic involvement (19 women and 24 men, mean age 41 years) have been reviewed. Three different subsets were recognized. The first (PS1), with predominant involvement of the axial skeleton, occurred in 22 (seven women and 15 men, mean age 39). The second (PS2) and the third (PS3) showed an overlap of spondylitis and peripheral articular disease. In PS2 this consisted of distal interphalangeal (DIP) arthritis (five women and three men, mean age 41), while in PS3 there was symmetrical polyarthritis (seven women and six men, mean age 42). Spinal involvement, present in every case, was characterized by unilateral and asymmetrical syndesmophytes, often nonmarginal and randomly affecting the vertebral column. Sacroiliitis, absent in the PS2 subset, was present in 15 of the PS1 and in two of the PS3 subgroup and was bilateral in six and unilateral in 11. The HLA-B27 antigen, absent in the PS2 subgroup, was found in 12 of the PS1 and in two of the PS3 subset. It was associated with sacroiliitis in 13 cases and with spondylitis without sacroiliitis in only one case. Nail changes were recorded in 30% of the total cases and showed a strict relationship with the PS2 subset (40%). Extra-articular symptoms, consisting almost exclusively of ocular involvement, occurred in three patients only (two cases of conjunctivitis and one of acute anterior uveitis). The clinical course of psoriatic spondylitis appeared less disabling than that of the idiopathic form.