OBJECTIVE To determine trends in the incidence and clinical presentation of ankylosing spondylitis first diagnosed between 1935 and 1989 among residents of Rochester, Minnesota, and in the survival of the patients. METHODS Population-based descriptive study. RESULTS The overall age- and sex-adjusted incidence rate was 7.3 per 100,000 person-years (95% confidence interval 6.1-8.4). The rate tended to decline between 1935 and 1989, but there was little change in the age at symptom onset or diagnosis over the 55-year study period. Overall survival was not decreased up to 28 years following diagnosis. CONCLUSIONS These data indicate that there is a constancy in the epidemiologic characteristics of ankylosing spondylitis and suggest that previously study results indicating changes may have been due to biases in patient selection and study design.