A family study of 2 probands, suffering from seronegative spondyloarthritis with ileocolonoscopic evidence of chronic inflammation of the ileum, is presented. On the father's side 2 brothers carry the HLA-Bw62 antigen. One developed typical Crohn's disease, the other showed chronic inflammatory signs of the ileum on biopsy. These lesions have frequently been demonstrated in reactive arthritis and ankylosing spondylitis (AS) with peripheral arthritis, in the absence of gastrointestinal symptoms. On the mother's side there were no clinical or histological signs of ileum inflammation, or spondyloarthritic features, although the mother carries the HLA-B27 antigen. The 4 children are all HLA-Bw62 and B27 positive, but only 2 of them have developed seronegative spondyloarthritis associated with histological evidence of chronic ileum inflammation. The familial aggregation of the seronegative spondyloarthritides is demonstrated. The lesions found on ileal biopsy in reactive synovitis and AS with peripheral arthritis may be related to Crohn's disease.