Prognostic factors for the occurrence of longterm disability and prolonged sick leave were determined in 182 patients with ankylosing spondylitis. A significant relationship appeared between functional disability (determined by the Stanford Health Assessment Questionnaire) and sex, age at disease onset, and the number of peripheral joints involved. Using a life-table approach, the probability of prolonged sick leaves was associated with peripheral joint involvement and work that involved carrying heavy loads. Long-term disability was more frequent after work involving exposure to cold conditions (relative risk [RR] = 2.01) and prolonged standing postures (RR = 1.34), while sedentary work (RR = 0.35) and formal vocational rehabilitation programs (RR = 0.57) seemed to protect against long-term disability.