A questionnaire relating to presence and nature of back pain was distributed to all 10,150 employees (59% men) of an industrial complex. The questionnaire was returned by 2892 subjects (65% men). Of these, 1880 (65% of responders or 19% of total) reported a history of back pain. One hundred twenty-four described their back pain as insidious in onset, persisting for at least three months, developing at less than 40 years of age, being associated with morning stiffness, and showing improvement with exercise. Three hundred sixty-seven subjects scored four of these five features. Pelvic radiographs of 342 persons were available for blind evaluation. Sixteen patients (12 men) were shown to have definite ankylosing spondylitis (Grade III or IV sacroiliitis or HLA B27-associated Grade II sacroiliitis). Only one of these persons was known to have spondylitis. The majority of these symptomatic patients had been seen by both medical and nonmedical practitioners.