Protrusio acetabuli (PA) is a complication of many disorders involving the hip joint. Its frequency and clinical features in seronegative sponyloarthropathy (SNSA) are unknown. The prevalence and characteristics of PA were studied in 50 adults and 25 children with SNSA (40 ankylosing spondylitis, 24 psoriatic arthropathy, and 11 Reiter's syndrome). PA was considered present when the acetabular line medially exceeded the ilioischial line by 3 mm or more in men, 6 mm or more in women, more than 1 mm in boys, and more than 3 mm in girls. PA prevalence was 25% in the total group (19 of 75 patients), 22% in adults, and 32% in children. There were no significant differences in seronegative disease type; PA was present with similar frequency between genders and did not correlate with disease duration, clinical severity of hip involvement, or previous medication. Radiological damage was similar in adults and children and did not differ in those with and without PA. Similarly, functional capacity did not differ between groups. It is concluded that PA is a frequent complication in SNSA; however, its presence does not seem to modify the functional prognosis of these patients.