A grading system involving six stages of arthritis from grade 0 = normal joints to grade V = extensive bony ankylosis in the sacro-iliac joints and a scheme applicable for quantitative registration of the radiographic findings of the spine in ankylosing spondylitis (AS) are detailed. These radiographic grading systems were used in a study comprising 48 patients with psoriasis (group A), 19 patients with AS and psoriasis (group B), 103 patients with AS (group C) and 231 first-degree relatives of the patients belonging to groups B and C (group D). Radiographic abnormalities of the spine were found totally in 80 per cent of the patients belonging to groups B and C. In these groups sclerotic anterior borders of vertebrae (SABS) and/or straightened anterior surfaces of vertebrae were seen totally in 66 per cent. SABS were earlier findings than syndesmophyte formation which was found in 60 per cent of the patients belonging to groups B and C. Except for ankylosis of the apophyseal joints and ossified interspinous ligament most frequently found in the lower lumbar region in patients with duration of disease more than 20 years, all abnormalities of the spine were most frequent in the dorsolumbar junction. Grade V sacro-iliitis was associated with ankylosis of two or more segments of the spine. Such spinal changes were infrequently seen in patients with grade IV sacro-iliitis. This finding supports the previous notion that among patients fulfilling the criteria for AS, there is a group with a non-ankylosing disease. Thus two different subgroups of AS could be identified. Except for frequent unilateral sacro-iliitis and slight changes of the spine in group A no radiographic differences were found between the groups A, B and C. Sacro-iliitis was found in 22 (9%) in group D, and 11 of those with sacro-iliitis had abnormalities of the spine compatible with AS.