Seventy-nine consecutive patients with active ulcerative colitis were studied to establish the prevalence and clinical features of articular involvement. HLA typing for A and B loci was performed. Forty-nine patients showed an articular involvement (62%). Three different clinical patterns were identified: ankylosing spondylitis occurring in 20 subjects; peripheral arthritis in 15; unclassifiable spondylitis in 14. When compared to the general population in our area, patients with colitis showed a significantly higher prevalence of the HLA-A1 (p less than 0.005), B21 (p less than 0.001) and B27 (p less than 0.05); among patients with colitis, those with arthritis revealed higher frequency of HLA-B27 (p less than 0.05). Our study reveals a high prevalence of unclassifiable spondylitis during ulcerative colitis, and suggests a new approach to the classification of seronegative spondarthritis.