['To describe the composition and distribution of total costs of ankylosing spondylitis (AS), and to identify predictors of high total costs among patients with AS.', 'In a prospective longitudinal study, 241 patients with AS reported information on health status, health care utilization, treatments, and work limitations on biannually mailed questionnaires. Annual direct costs were estimated on the basis of reported ambulatory care visits, hospitalizations, diagnostic tests, medications, assistive devices, nonallopathic treatments, travel to visits, and paid household help. Indirect costs were estimated from the number of work days missed or, for retirees and homemakers, the number of days of activity limitation. A similar analysis was performed for cumulative costs over 5 years in a subset of 111 patients.', 'Annual total costs averaged $6,720 (in 1999 US dollars; median $1,495). Indirect costs comprised 73.6% and direct costs comprised 26.4% of total costs, although only 95 patients (39%) contributed to the indirect costs. Functional disability was the most important predictor of high total costs. The likelihood of having high (>$10,000) total costs increased by a factor of 3 with each 1-point increase in the Health Assessment Questionnaire disability index modified for the spondylarthropathies (HAQ-S; range 0-3). Results were similar in the subgroup of 111 patients who were followed up for 5 years, among whom the likelihood of high cumulative total costs (>$50,000 over 5 years) increased by >6 times with each 1-point increase in the HAQ-S.'] Does functional disability predict total costs in patients with ankylosing spondylitis?