This study was carried out to determine the incidence and to describe the radiographic appearance of ankylosis capsularis ossea at the canine sacroiliac joint, as well as to determine factors, which might cause radiographic misinterpretation of this finding. Bone specimens of 646 pelves of pure-bred dogs were assessed for morphological evidence of ankylosis capsularis ossea. The lesion was most commonly present at the cranioventral aspects of the sacroiliac joint. More than 50% of adult dogs had signs of being at least mildly affected. Among these, large and giant breeds were most commonly involved. Unlike moderate and severe degrees of this disease, smaller osteophytes had not been visualised on radiographs. For radiographic diagnosis, angled views of the sacrum are recommended. In central views of the sacrum, the presence of increased sacral concavity consistent with increased ilial convexity of the caudal aspect of the sacroiliac contact area might simulate the presence of ankylosis capsularis ossea. The clinical importance of the present findings is discussed in context with human literature.